It depends entirely where you are. I've had T-Mobile since the original G1 (first commercial Android phone) came out in NYC and have had no issues. Service is a bit spotty on the train up to Albany/Rochestor, but it is with all the carriers. The only places you have trouble with T-Mobile is when you get in rural areas, which I usually don't. The one exception is part of my family we visit and only Verizon has coverage by them. In that case, I just switch to wifi calling at their house. Or turn it off for a while.

In the states, if you're obsessive about your phone being on every minute of every day (and, realistically, anyone actually that important... as opposed to thinking they are for their startup... is still forced to carry a pager), you basically have to go with Verizon. That's why I have a Verizon hotspot as a backup. Not as a backup for T-Mobile, but as a backup for Time Warner, since it goes down at least once a month for a little while and down for a full day every few months.