Don't get me wrong, it does run smoothly, I just noticed several instances where scrolling through a list or a map would see a drop to 30 fps rather than 60 and plenty of little hitches while scrolling around were common as well. It responded much faster than the Nexus One, that's for sure, and is much better than the original Galaxy S phones (which, despite faster hardware, seemed to run slower than the Nexus).
I'm unsure which ROM was being used, but the device had been rooted.
There are definitely hiccups on the iPhone 4. I loved that phone, but it was not always completely smooth once you started getting into a lot of different tasks. The iPhone 4S completely corrects all of that, however, and is about as perfect as I could want. All tasks perform faster and there are literally no skips or hiccups anywhere thus far. It solved all of the issues I've had with the 4. It basically runs like an iPad 2 now.