Well, it's true that you can roam on Cingular but the catch is, if the signal is weak on T-mobile, it will still try to connect to T-mobile. It will not use Cingular. The only time you can roam on Cingular is if T-mobile have no coverage in that particular area.
Don't rely on the coverage of Cingular. You should always check the coverage on T-mobile.
If T-mobile says you going to get one bar than that's what you are going to get. That's already pretty weak so you may want to go back to Cingular.