I've had no problem with 1900mhz reception on my 903t. Just picked it up today, but it's worked, so far, from San Gabriel, through downtown LA, worked brilliantly in Little Tokyo (which is only appropriate, after all!), and up to my house in the Hollywood Hills.
Notably, I get better, clearer, reception at my house with my 1900-only 903t than I ever did with my S710a! With the 710, I could only make/recieve calls clearly if I went outside onto my back deck (which has a clear line-of-sight to cell towers), but with my 903t, I can clearly hear calls from inside my bedroom and hallways. It's using Cingular's network, too, not T-Mo. Maybe the 1900 reception here is better than the 850, but the S710a was defaulting to 850 because it's cingular's preferred band?
So, if you're in a major metro area, and don't plan to be out of developed areas often, or have a backup phone with 850, there should be nothing wrong with having a 1900-only phone.