I had the "incorrect missed presses" issue maaaaybe one in a blue moon. Other wise, I've had no problems with Mango aside from the battery life is been a bit lower. I chalk that up to using the phone a lot more though.
No sluggish scrolling at all. Using a T-MO Venue Pro but with a non factory 16GB SD
I haven't received mine yet so I can't say for sure, but based on user reviews I've read on the net, people that bought a DVP with all firmware updates and Mango pre-loaded seem to have great experiences wih their device. Perhaps a fresh install will make it run better.
I have had a an issue with the keyboard not resizing correctly in landscape. Its not a major issue (switching to portrait and then back to landscape fixes the problem). I don't know if this is a Mango issue or not because I updated as soon as I got my DVP.