I'm a N800 newbie who bought the device with a view to using it for media playback. I've been playing with some media players and my thoughts are:

Kagu: Seems to be a total resource hog. My N800 grinds to a halt using it.
Canola: Has some issues with some of my Lame encoded mp3s. Keeps repeating them instead of moving to the next track. Seems to have lots of lag.
Std media player: Seems to be most reliable of all, but it's a bit crude and basic.

I need to play and with these a bit more, but I'm pretty disappointed with what's available compared to that for my Palm Treo.