PI use Verizon with my Business Treo 700w and T-Mobile for my personal Cell. I like T-Mobile but as Andrew said, they could have better coverage. I placed my vote for Verizon because I like the phone I use with them a lot more than my rinky dink Nokia on T-Mo.
PPVerizon, technically, has the best National Coverage for the US and their new EvDO data network (which I use because I don't have to pay for it!) ROCKS!!!