The XBOX gives off a really poor RGB signal, particularly when compared to the PS2.
I used to have mine hooked up to a Sony RGB monitor and the quality difference was quite stark. Mine also had some weird issues with showing extremely bright/white color schemes, and would start to flicker. Don't know if Commodore monitors have the same issue, or if this was a problem unique to my monitor.
If it's your only option (and you don't want to use multiple monitors for all your consoles), it's acceptable, but component is better.