Now that I've had my Google Nexus 5 for a while (opened the box the last week of November, and activated it on T-Mobile mid-December), I wanted to write out my impressions of it, for anyone interested (partly because I don't want to feel like I got posting privileges for nothing). I will start with saying I'm an Android fan, and this will be a very amateur review where I won't be very critical, and this will mostly be me praising stuff.
Here is what I have:
32GB Google Nexus 5 (Black) - purchased directly from the Play Store (you can see the specs from Google here).
After opening the box, I followed the instructions provided on Android forums to unlock the bootloader and root the phone, using ADB interface. And being a Nexus phone, it was all pretty easy, the hardest part was getting the right driver installed on my PC (Windows 8). After searching around for ROMs, I landed on Cataclysm, which is nearly stock with a few extra touches.
Once my contract with Verizon expired (mid-December), I made the carrier switch to T-Mobile.
My previous phone was a Galaxy Nexus, running Shiny ROM (stock with a few mods) with Android 4.3.1 (it's up to 4.4+ but I haven't updated it) on Verizon Wireless.
So far I love the Nexus 5, it's got a nice big (4.95") screen with 1080 HD, runs pretty fast, and this could because I'm coming from a Galaxy Nexus on Verizon but the battery life is really good.
One of the benefits of that 1080 screen is that it opens up the HD options of some apps. Like Comixology. I didn't get that HD option on my previous devices.
And here's a shot of the cover in HD (not sure I can tell the difference, but maybe you can).
I also took advantage and downloaded the highest resolution version of one of my current favorite games (Tales of Illyria).
The phone is lighter than the Galaxy Nexus, despite being a little bigger, but still has a good solid feel to it. The camera isn't the greatest - I generally have to use the tap-to-focus for every picture, but as long as I do, they turn out just fine. The speakers probably aren't as good as the One X, but they're good enough.
Bringing up Google Now by saying "Ok, Google" was a cool trick, but that's really all it was to me, so I've continued to use Nova Laucher since I find the custom gestures available with Prime to be more practically useful (that voice activation only works with the stock launcher).
My OTG cable still works, so I can play videos or music off a flash drive, and the 32GB is plenty for everything else. It also works with an HDMI adapter, but if you had an MHL like I did, you'll need to get a Slimport MyDP for the Nexus 5.
As I mentioned, it's mostly stock, but Cataclysm ROM offers a few extras, like additional reboot options (normal, recover, safeboot) and more, than you can access through the settings, which gets added to the main settings menu.
From there, you get a few options. My favorites are centering the notification bar clock, and changing the battery indicator to text.
And more in the Misc. section
As I mentioned, I made the switch from Verizon to T-Mobile. Mostly because I wanted the Nexus 5, and Verizon was the only carrier that wasn't going to work with it. But also because Verizon is a lot more expensive.
Living, and working, in the Los Angeles area, I've found T-Mobile's coverage to be pretty damn good, there's even areas where I'd lose my Verizon signal that T-Mobile works in.
I went with the $50/month plan with unlimited talk, text and 4G that gets throttled after 500MB. That doesn't seem like much for data, but I have WiFi at home, work and the homes of the friends and family I regularly visit, and Los Angeles has a lot of TWC hotspots, so it's not that big an issue. My Verizon bill was around $70/month thanks to a corporate discount and even then T-Mobile costs a good amount less.
Switching over was mostly painless. At the time, the Nexus 5 wasn't on the "bring your own phone" list, so I had to call in instead of using the website, but otherwise, it went pretty smoothly. I got a temp number to use until my regular one switched over from Verizon (I made sure to provide my account number and not just my phone number after reading on a forum that they need both), and they said up to 24 hours but it only took 2 or 3.
The Nexus 5 doesn't include a native messaging app, unless you count Hangouts, which didn't seem to work too well for MMS and group texts, but 8sms works pretty well, so I'd recommend that. It does a great job as a stock messaging app and it's free.
I also had some trouble getting Google Voice set as my default voicemail service, but eventually I found some helpful forum advice: setting the forwarding numbers to the one listed for Google Voice (in dialer settings) and then rebooting.
So there you have it, about 2-3 months in, and I still love my Nexus 5 and am very happy I switched from Verizon to T-Mobile. And if you're looking for a mostly stock ROM but with a few extras, I recommend Cataclysm.