Buster,
I live in SoCal (South Bay) and travel all over the US, Europe, and Asia. I used T-Mo for 3 years, and recently switched to the new AT&T GSM America plan with a quad-band v600. This combination really suits me well, and I have yet to drop below 4 bars of RF reception in my travels. I believe this is both because AT&T GSM service is improving (at least in the major markets) and the fact that I'm using a quad-band phone which can access more towers of more carriers.

I was very sceptical moving from T-Mo to AT&T, but I'm glad I did!! T-Mo was a real PAIN for me, and I hardly got any RF with the t68i (and later t610) I had.

Spent $30.00 unlocking the v600, and since it's quad band it works everywhere in the world I've been, with a local SIM card. All the best!