Pros: Good screen, good battery life, great speakers, has new T-Mobile bands, cheap, can add SD card, double tap to wake/sleep (working much better with recent update), near-stock Android experience. Looks OK, doesn't look/feel cheap. Light. Unlocked. Many of the pre-loaded apps can be uninstalled.
Cons: Occasional lag (not such a big deal to me, nothing like my old Galaxy S3), sound a little muddled on phone calls, replacing stock launcher (I've tried Nova and a couple of others) removes double tap to wake / sleep. Might be a little too big for some, but bezels aren't huge. Photos not so good in anything but bright light. Idol 3 logo on back might be a little goofy for some. No T-mobile wi-fi calling. Only 16 GB onboard storage (can move apps to SD card).
TBD: durability
The cons, to me, are nit-picks. The solid "con" to me is the camera, it really needs good light. It's a great phone for me, and if I break it / it gets stolen, it's not such a huge loss.