All depends on the codec, bit rate, etc.
Converted for use at the correct resolution, videos on the 5800, 5230, N97, etc. look easily as good as they do on the iPhone & iPod Touch.
I used to use the Touch for videos, but since it died, and now having a 5230 ... converted some videos for it, and they looked pretty darned excellent.
Youtube is a different matter entirely though - the 5800 client seems to be trying to use the lowest resolution and bit rate possible. Its effectively pointing to m.youtube.com