HD quicktime video is a little choppy on my setup, but I haven't had any issues with SD quicktime videos I made using Handbrake from a DVD (Apple Universal). Full screen Netflix and Hulu video is choppy, less so when not full screen but still exists. (Not sure if it is because of the wireless G or simply flash & silverlight suck on this processor.)
I have a 2 gig ram, with a stock Dell 32 gig drive. (Quicktime videos played from the hard drive.)
I have only had the computer a couple of days, and have not had time to watch a long video, so this is just my initial impressions.
Trying to encode a iphone version from a hd version with Handbrake is very slow (2 hour 720p video was showing 25 hours to convert to a iphone compatible quicktime SD movie.) My plans are to try and use a turbo .264 USB processor to convert and watch some auto races using the dell 9 mini while I am on the road.
(Most of the F1 races are only captured in HD now, I am hoping to get them via bittorrent and then convert to SD to watch on the mini 9. I hate finding out who won before I can see the race myself.)