I switched from Verizon to T-Mobile about 3 years ago. Never regretted it.
The switch was brought about my Verizon's utter incompetence when I wanted to switch plans, add two lines, and upgrade handsets. The guys on the phone had no clue what I was talking about, and the guys in the store were feeding me all kinds of BS
about prices and costs and "re-activation" fees. So, after a month of frustration and the poorest of poor customer service, I decided to switch.
I did the same as you; researched the other major carriers. It came down to a choice between Nextel and T-Mobile, and T-Mobile got it because at the time they were the only carrier who offered nationwide no-roaming and, being GSM, I could use my phone in other countries.
T-Mobile for me have been nothing short of fantastic. They had a nice choice of plans and helped me choose the one that suited my needs, and they have a good range of handsets. When it came time to upgrade my handset (just a couple of months ago), I got a T-Mobile Dash for a ridiculously low price because of my good standing and the fact that in nearly 3 years I hadn't upgraded my handset. They also upgraded my wife's handset for free.
Whenever I've had questions or queries or wanted a service added to my account, it was hassle free. For example, when I flew to London last February, I realised as I was standing in line to board that I hadn't activated International Roaming on my account. I called while I was in line, requested it, and when I landed in London and switched on my phone, I was roaming.
Also, I took my number with me from Verizon to T-Mobile. They handled the entire thing, and although they said it would take up to 3 days to port my number, it was switched to T-Mobile in a matter of hours rather than days.
T-Mobile will have to mess up in a major, major way for me to consider leaving them. Call quality is great, I don't recall ever having a dropped call, and their customer service is second to none.