Phil over at android central did a video of the DX and from what it looks like it seems to be very smooth. He did alot of jumping around in the phone and it took it all and smiled back asking for more. Looked pretty smooth and snappy IMO.
I've watched a few videos too, but I have noticed what looked like lag when scrolling through the launcher or flipping through homescreens really fast. Those are really the only times I've seen it not run smooth, but that makes me worry about the other parts of the phone, like browsing the internet or touch responsiveness. BTW, I've updated the OP with links to more reviews mentioning the slowdowns.