I loved my original Moto X, which I bought used for an extremely reasonable price. But it had a few flaws:
But it was the last phone ever made that was less than 2.7" wide, which is kind of the cut-off for one-handed use for me. And it had OLED.
So two weeks ago I bought a Nexus 5X 32GB with Google Fi for $250.
The incentives to upgrade are now much less than they used to be. A flagship from 3 years ago is still a very usable device. So I'm pretty interested in the changes.
One-handed operation: I could grip the Moto X with one hand, but instead for the 5X I have to balance it on my fingertips at the center of the phone. The only reason I consider this acceptable is that really one-handed operation on the Moto was not awesome. And truly, I have already gotten used to this gigantic Nexus. It actually sits in my pocket better than the Moto did, because it's so big it can only orient one way.
Screen: On the one hand, the increase in DPI is surprisingly noticeable - fonts are *smooth* on the Nexus 5X! And the Nexus is actually much more readable in direct sunlight. On the other hand, LCDs suck! AMOLED really spoiled me. LCDs have finite viewing angles, and no black! I used to read at night with a very dim white font on a deep black background, but now I get a brighter gray on top of a bright gray, and the phone is so bright it casts a beam on the wall, and when it's dim the color distortion from viewing angle is much worse. Lame.
[update 2016/06/30: One thing that sucks is that the ambient light sensor (adaptive brightness) is glitchy. I will be looking at the screen comfortably, and suddenly the brightness will change to something almost unusable for a few seconds, and then change back a few seconds after that. I have verified that I'm not covering the sensor with my finger or something. The really frustrating part is that when it works, adaptive brightness is actually fantastic. It adjusts to direct sunlight and dark rooms much better than the one on the Moto X - it has control over a wider adjustment range, I think. So I'm not eager to accept the obvious work-around of disabling it.]
[update 2018/08/20: In August 2016 I wrote my own app to manage screen brightness and it has been satisfactory ever since. It turns out a lot of Nexus 5x devices, including mine, shipped with defective light sensors that tend to read off-scale low inappropriately.]
Performance: So far, the Nexus is totally smooth and flawless. What you would expect out of a phone designed to run on the OS it is running on. Presumably Google will push Android Ney Lime Pie onto me soon, and the Nexus will be slow like last year's phone. *sigh*
[update: 2017/02/09 I accepted the auto-update to Nougat a long time ago and it has not caused any detriment except for a continuing inability to stop futzing with the look and feel of the stock dialer app.]
[update 2018/08/20: I don't know when exactly it happened but for about 6 months I notice, it is slow. Not ridiculously slow, and not slow at intensive things. I think mostly it's low on RAM, using any app causes all of the background apps to be low-memory-killed and then it has to load everything from scratch, including the task switcher! In other news, my phone overheats dramatically in most video games and then goes into a slow mode where it cannot play the game.]
Camera: I'd always thought of the Moto X camera as good enough, but the Nexus 5X is loads better! It is a little grainy if you zoom in, but generally every picture is pretty good the first time you take it - even indoors! I no longer take 5 snaps of each photo in hopes that one will turn out. Also the camera app is much faster. Also, at least on the Nexus, Google has really improved the UI on the stock camera app. A real improvement for capturing the kids. "Take a camera!" [of my back as I run away from you].
[update 2018/08/20: The camera is the only part of the phone I still find to be absolutely flawless.]
Fingerprint: This was a real pleasant surprise. Subjectively: I'd say the fingerprint scanner has a 0% false positive rate - it never unlocks in my pocket! It generally wakes up completely in less than a second from when I touch my finger to it. It has about a 20% false negative rate - sometimes it doesn't accept my fingerprint. And about 3% of the times, I am unable to wake it with a fingerprint at all and must use my PIN (causes vary). That is a huge improvement over the "slide to unlock" screen, which woke up in my pocket about once a day, and was not reliable about registering my impatient slide, and which took at least 2 seconds every single time. I have been saying, "fingerprint technology is not ready, it is a useless novelty," but that is not true anymore! The only thing it never likes is a sopping wet finger, but a quick wipe on my shirt solves that.
[update: 2017/02/09 A long time ago I added an additional pair of fingerprints for my index fingers immediately after taking a shower, and it got even better.]
Marshmallow: *shrug* There are a few changes I've noticed, and I like them all. But really it's hardly different from Lollipop.
Google Fi: Easiest experience I have ever had porting a number! The hand-offs seem pretty flawless, I've never noticed it happening and I don't seem to be dropping incoming texts or calls. The network is only T-mobile and Sprint (?), so it doesn't cover rural areas as well as AT&T did. *shrug* The price can't be beat! The only bad thing is that it seems to sometimes have the unique T-mobile flaw where the audio quality is *severely* degraded on calls to call centers and voicemail. Inexplicable.
[update 2018/01/30: Google Fi is now my least favorite thing about this phone. It's a mixed bag. Sometimes it picks the worst of the 3 available connections, enough times that placing calls from home is a nightmare -- it picks one of the 2 good options often enough that I don't get in the habit of forcing the issue, but picks the 1 bad option enough to be a severe headache. Also, it doesn't work at all in rural areas -- T-mobile and Sprint aren't complementary carriers, they both have tiny service areas. And finally, it says "Sent." on text messages that haven't been sent yet, which is pretty awful.]
Battery: The battery generally lasts longer than my aged Moto X under light usage, but it goes through it faster if I'm playing video or games. Not very impressive, but quick charge is a very effective mitigation! Now if I plug the Nexus in for 10 minutes here and there throughout the day, it stays near 100% charge all the time. My Moto had trained me to plug it in at every chance, and even so it was usually hovering around 50%. Vast improvement!
[update 2018/08/20: Battery is still good enough! Sometimes I go two days without charging!]
Speakers: It's nothing to write home about, but the Nexus 5X is pretty loud. And I like front-facing better than rear-facing. As an aside, Google Now voice recognition seems more reliable than it was.
Google Play Music: Google finally made one of their add-on offers so easy to redeem I actually got it (somewhere I have 10GB of Google Drive storage for 10 years waiting for me, I guess). Google Play Music is usable, I suppose. Much more usable than Spotify and Rhapsody were! But it still sucks because the catalog is so small, and because my phone is the only device that will play it. It came with YouTube Red, which seamlessly forces you into a Pandora clone if you forget to actively stop a music video. That's probably why the industry is so mad. *shrug*
[update 2016/06/30: It also sucks because the curation is crap (it has albums with 0 tracks!), it shows 83 albums for every artist (hard to find the 9 albums they actually made), the album list is insanely sorted, it has so many distinct modal displays that you never know what the result of a given tap will be, the interaction between "queued" and "played" tracks is awful (though at least it's predictable), it skips sometimes (!), it plays tracks out of order sometimes (!). Really its biggest weakness is that there is a single app that you have to use to access the service, so naturally that app has to accomodate all kinds of users, and thus can please none of them. There should be a diversity of apps, then it would be possible that there might be one that is targetted for people who think in albums (me).]
OK Google (update 2016/07/31): One of the features proprietary to the Moto X was the ability to activate Google Now hands-free. Without even pressing the wake button, you can exclaim into the room, "OK Google". I didn't find it to be very useful, because it often did not wake, and if it woke, it would not understand. The Nexus 5x (and accompanying software), however, makes it useful! I can say, "OK Google call wife on speakerphone" and it does it! Very reliably! The only downside is I have to unlock it (PIN or fingerprint) to get it to proceed, but it is still often more convenient than doing it manually (especially because I can fingerprint-unlock without looking at the phone). The other downside is "OK Google end call" doesn't work to hang up. But what it does, it does reliably, and now all Google needs to do is add features.
update 2017/04/14: Ugh, right when I was really getting to find OK Google useful, they took it away. Now OK Google (or long tap on home) brings up the Google Assistant, which is useless. The voice recognition is not good enough to compensate for the fact that I'm irate every time I use it. Assistant often says, "I can search the web for that if you want." I don't know what I'm supposed to say to that, but "what the fuck do you think I was talking to my god damn phone for you stupid fucking shithead" doesn't work. And there is no keyboard interface (useful for the long tap on home). And it is unwilling to do *anything* without unlocking the phone, completely removing the point of having always-active listening. I did not manage to have a single positive interaction with it before disabling it, and I tried on several occasions. It took me three tries just to get assistant to tell me there really is no keyboard interface. An epic mis-step, IMO.
[update 2018/08/20: Google assistant keeps changing but is still unusably bad.]
update 2017/12/20: I broke (and replaced) the Nexus 5X screen in May, and combined with a few other things I have been using my old Moto X some. I have to say, my long-term impression is that the Moto X is superior in every way except battery life, charge time, and camera. For example, many videogames can't run for 5 minutes on the Nexus 5X without overheating, but they run fine on the Moto X (no perceptible speed difference). And OLED is really good.