I've got it on my 3G. I'm not impressed with the performance. The new features are impressive, yes, but after installing iOS 4.0, my iPhone is acting like it's had its RAM sliced in half. The UI is nice and snappy immediately after a sync or restart, but soon after it becomes laggy and almost unusably slow. I've had far more apps crash than I did under OS 3, and the OS is so bad at memory management now that music playback from iPod.app will start stuttering just from trying to scroll a page in Safari.
I really hope this isn't the final GM build, because at least on the iPhone 3G, it needs a LOT of work. I'm getting an iPhone 4 next month and giving my iPhone 3G to my wife, and I really hope that by then the thing runs better than it does now, because right now it sucks.