My personal experience with the Curve on at&t was about the same. This was on the island of Oahu. Mail was ok but the browser was taking way too long.
Last month I picked up an 8320 at T-mobile to try out on their 14 day plan to see how the UMA worked overseas and if it would be worth the switch.
What I didn't expect was the speed of T-mobiles EDGE. I was shocked and tried it next to my at&t Curve and the difference was like between dial-up and broadband. This might be because of the area that I was using it.... not sure.
When I roam overseas on EDGE the speed between the two was the same. Of course wifi is really fast on the 8320...