Ah, Max... I was referring back to the OP. My bad.

Further to Max's video: I don't see much (if ANY) difference between Vimeo and the hosted versions.

The only stuff that looks bad are the zooms.

Just an update. The very jerky (and totally unwatchable) video playback I was seeing on my videos on YouTube HD as reported back in April seems to have "cured" itself (having watched them again tonight for the first time in several months). They again play back (in my eyes!), typical web playback for 25p footage/superbly well in HD - exactly as they did when I first uploaded them many months ago - and some of my stuff is very challenging for Flash.

This suggests this was (at least in my case) a temporary YouTube/Server/Local issue which now seems to be resolved. Might start using it again now!

I know this thread is almost a year old, but I have been suffering with bad jerky playback of Youtube HD videos on my Dual PowerMac G5 which plays smoothly on my wife's Intel iMac.

I thought the G5 processor was not able to decompress the files fast enough to play it back smoothly.

After reading scouring the web for some hints:

PowerMac Dual 2.0 can't handle YouTube HD video. - Page 3 - MacNN Forums

It turns out that Adobe's Flash player performs poorly on my G5.

If you opt into Youtube's HTML 5 beta... then HD videos [with no pop up advertisements] will play fine in Safari.

YouTube - Broadcast Yourself.

Go figure.