Hey folks.

I've been playing 700MB divx .AVIs on my N810 using mplayer.
I ripped them using Fair Use Wizard at default settings.
Those movies/videos sometimes seem like they are on the verge of skipping, but I've found that if it's the only task my tablet is performing it will settle down and play smoothly.

The .flv stuff from YouTube is flawless, but it's also much lower resolution video, and often in the square aspect ratio as well.

Overall it isn't as strong as I'd like, but it still works fine and hasn't annoyed me yet. (always a good arbiter)

What blew my mind right away was the mYTube application.
It does stuff I was using a Firefox plugin to do on my PC, and in a FAR more refined and elegant manner!

I think the main thing to remember is to make sure that playing video is the only thing your tablet is doing when you are doing it.
That way it can use all the resources available to it to do the job.