My biggest issue with T-Mobile is
1. Building penetration. I get edge inside my house and 5 feet out the door I get LTE. This is the case in most buildings
2. Call volume is crazy low when reception is bad. This has caused me a couple embarassing mixups because I couldn't hear the caller.
3. Most of the time it's on HSPA/HSPA+.......but those speeds are still pretty good so it's not a big deal.
4. When reception is bad (i.e. in my house) and you are texting long conversations there is often a huge delay and if you have too much back and forth its starts coming in and sending out of order (really annoying)
I have basically given up on the call and text part and I am just using Google Voice and Hangouts for that. 4G data connection is reliable enough outside, and most of the time if I am inside somewhere with bad reception I can use wifi.
The pro is data works pretty well when I do have reception. Basically every network level from Edge to LTE has better speeds than ATT at the same level.