I had the same sort of experience as Fatbike. We had all Nextel 7510's and 7520's and my 7520 had given up the ghost once shortly after I got it, so we had it replaced. well, the radio went out in it about 3 months later (still within the 1 year period, but after the merger) and they told me it would cost us $35 and I would be without my BB for 2 weeks because it was past the 30 days since it was last replaced. I told them to go fly a kite. I pulled out an old 7510 to use while I researched other carriers and I am happy to say that I just ported over our last nextel line to t-mobile this morning. I almost died of ironic laughter when the Sprint rep suggested I upgrade the nextels to 8830's. I tried to find a polite way to tell them they were overpriced and their CS was crappy. I don't think I succeeded. Oh well
I used to get all sorts of hang-ups while on the the line to CS then I'd have to call back and the reps were mostly idiots who seemed to revel in making my job as difficult as possible. Mind you, this was after the merge with Sprint. Before the merge, I had pretty decent experience with Nextel.
I really like t-mobile. we have had a few BB's go bad and they had them replaced with no downtime for my users and little hassle to me. Although, it helps to be a business account - we get the really helpful CS agents. But I have t-mobile for my personal lines including two BB's and they have always been very capable.