I have a Nexus 4 (bought it when they dropped the price by $100 just before discontinuing it). The Nexus 4 and Nexus 5 have a lot power for the price, but there are a few things that they do to get the price down, and while it wasn't enough of a concern for me to stop me from buying (16 GB for $249), it could be for others:
(1) -- no user-serviceable battery. If the battery craps out, the phone is a brick (well, it *can* be taken apart to replace it, voiding the warranty and comes with its own risk of bricking the phone). And it also means you can't keep a spare charged up battery to swap out, or use a higher capacity battery for longer times between charges.
(2) -- you can not attach external USB storage devices to Nexus 4 and 5 phones. You would need to find some way to do this with Bluetooth, most likely, and some equipment that can be hooked into most Android phones can't be used on a Nexus 4/5.
(3) -- No expandable storage with an SD card. In this case, 16 GB is a hard limit.
That said, I'm not enough of a power user that these were big problems for me, and these are some of the tradeoffs the designers made to get the price down. These are design issues that have a lot of the critics grousing, but they have to ask whether those features are important enough to warrant paying another $200-300 for something else, such as a Samsung Galaxy. In my case, I could justify $250 but not $500.
I'm using my Nexus 4 with an AT&T-based MVNO (H2O Wireless) and have had no problems at all. And yes, these phones are absolutely unlocked. You can change providers at any time as long as the phone is compatible with their service. This is important since most people who want unlocked phones are specifically looking to avoid contracts.
"Hey, for every ten dollars, that's another hour that I have to be in the work place. That's an hour of my life. And my life is a very finite thing. I have only 'x' number of hours left before I'm dead. So how do I want to use these hours of my life? Do I want to use them just spending it on more crap and more stuff, or do I want to start getting a handle on it and using my life more intelligently?" -- Joe Dominguez (1938 - 1997)