Meh...its all relative.
My T-Mobile phones get better/constant signal at my new job site, where every other carrier struggles to get signal/connection, including my AT&T phone. All of my co-workers have to leave the area to make a call. However don't forget to factor in the phone itself...my $300 (full price) T-Mo branded Motorola Defy consistently has 10-15dBm better signal than many "flagship" phones, on either T-Mo or AT&T via UMTS.
Then you have a situation like Verizon, where I'm 2 miles from (2) different Verizon stores, and (5) stores/(2) corporate office buildings within 8 miles, and I'm in a weak/no signal area. Yet my T-Mo phones struggle inside my local supermarkets (full signal in parking lots), when every other carrier is issue free.