I've used T-Mobile in the past, about five years ago, and at that time it was, frankly, crap. However, I've gone back to them on a sim only 30 day contract in the past month, and I've been impressed by how much they've improved - much more extensive 3G coverage than o2 or vodafone in the area I live in and, importantly, I've had no dropped calls - something which was a curse with them before. I think this is to a large extent due to the site share agreement between Three and T-Mobile, which has greatly increased their coverage in areas they didn't cover that well before.
Its worth saying too that T-Mobile are tying up with Orange before too long, and apparently one of the first ramifications of that is that T-Mobile and Orange customers will be able to roam onto each other's networks, thus increasing coverage further. Of course the overall implication of this is that T-Mobile 3G will eventually be making use of the existing T-Mobile network plus the Three and Orange networks.
After years of them being largely in the wilderness anywhere outside the 3 main cities, I think T-Mobile is probably a pretty good network to be on at the moment.