On my 3 year old laptop with Intel graphics, I can run compiz or not.
Compiz runs ok (apart from the occasional hard lockup, and crashes during video playback, and... all of which will surely be fixed eventually, but rule it out for regular use right now).
It seems a bit slower to display things with Compiz on than off, but usable both ways.
But I'm sure it uses a more battery when Compiz is used. That's my main reason for turning it off. The computer just feels like it's working harder with Compiz on, and you can see the redraw rate.
I found all the *useful* features of Compiz (like how it lays out windows and lets you move them) would work equally well with 2d acceleration, especially if there's 2d scaling. The 3d effects are pretty but I didn't find any of them useful; neither do I find translucent windows useful - just harder to read.
So I turned it off to save battery and make everything a bit smoother.