Really? I was in exactly the same boat... was with three, wanted the desire and the best deal was on tmobile (18 month, free phone and 25 a month contract) which they couldnt match.
While i love the phone, comparing t-mobile to 3 the former company is far worse than the latter. 3's customer service was excellent...if you had a question or a problem you called 333 from your phone, got through to someone almost instantly and 9/10 they gave a resolution there and then. The couple of times i had a faulty phone, they even came and picked the handset up from me and then delivered it back the NEXT day. From the 6 years or more i was with 3, they were exemperly with there service.
Tmobile? I've tried twice with there customer service number and both times couldnt figure out the options i needed to press to even speak to someone. Plus my signal is generally quite weak.
If i could have had the same deal i have now on 3, i would have stuck with them in an instant. Just my opinion though i guess