Man, I'm amazed. I don't have a lot of experience with Android phones, but the Aria is a great phone in my opinion.
See, I waited for Android phones from ATT as I'm stuck with them due to my employer. Finally, the Backflip was announced and I got one pretty early. Well, what a piece that thing is. Constant instances of unresponsiveness, poor data transfer in the areas I frequent. Very bland interfaces for the setup and utilities. The Aria runs all over the other unit. Responsiveness is very good, data transfer while sitting at my desk, which was impossible most of the time with the backflip, is seldom unavailable and the UI provides more desktop space with 6 instead of five (not sure what to do with it all yet). Heck, as a test I loaded Tux Rider on the unit. I had loaded it on the backflip, but it was jerky and unplayable. On the Aria it was smooth and fun.
Like I said, it is night and day difference. I'm actually quite glad, as I love the size of the unit. It fits quite comfortably in the pocket and the keyboard though small, can be worked with one hand quite well. Only complaint is the battery, as I use the unit to play audiobooks throughout the day. It seems the battery drains pretty quickly. In comparison to the backflip, the backflip seemed to last a bit longer, but not by much. I'd give it an extra 10-30 minutes of runtime while constantly playing audio.