After a couple months, I'm reasonably happy. I bought it for the incredibly low price as a way to try out Windows Phone and T-Mobile (coming from Blackberry and then Android on Sprint). My problems with reception have been limited to T-Mobile coverage issues, though the Wi-Fi calling helps make up for that. Battery life has been excellent--much better than Nexus on Sprint, though not quite up to the Blackberry 8330 or 9650 (which isn't a fair comparison). I'm not a heavy user and while I charge it every night I suspect I could get two days with my light use. I don't have any complaints about lag, but that's in comparison to Android, not higher end Windows devices. My primary complaint is that lack of flash--not so much because of the camera as because I like having a flashlight app. My other big complaint is the lack of a notification light or customizable text/vmail alerts, but that's a WP issue, not a phone issue. (I'm also annoyed that it takes two steps to answer an incoming call, and that the phone dialer seems to default to the call log rather than to the dialpad. Why?? Am I missing something.)
In short, for a non-power user who appreciates low price and long battery life more than all the bells and whistles, it seems like a great phone. I've been thinking about getting a Nexus 5, but I'm just not sure it's worth the extra $$$ (for my needs) and I really like having a removable battery.