I've had problems with realplayer for years. Think others might as well?
To be fair, the biggest problems I have had are with realplayer. The videos are stuttery and drop frames. I seem to have found a bug - the contrast of a video seems to be flashing and alternating during playback. Its very annoying and distracting. Only noticed this on the air tho, not had issues on my mac mini (gma950) or my mac pro. (Disabling overlay seems to help, the flashing stops - but at the expense of poorer, un-antialiased output).
Most other videos seem to be ok, at least standard definition ones don't drop. Most of my divx and xvid files are fine. Even short viewing times (like 20-30 mins into a movie) and the fan will spin up to max. And it seems that streaming over a network makes it slower as well, taking up cycles of CPU time. The same file from the 80GB hdd isn't as jerky as streaming over 802.11G from my mac pro)
Not tried 1080p trailers yet, but most of the 720p ones are smooth. There are a few which have issues with frame rates - curiously "where in the world is osama bin laden" is particularly bad and unsmooth.
And I now seem to have encountered a high pitched squeal/buzzing when connected to headphones. Dunno if its my dud machine or a generic defect like the similar issue in macbook pro's?
Overall I love the form factor and using it for surfing a music, but for even basic - middling levels of video/HTPC use and it falls short. Kinda disappointed.