The laptop I bought is a G4 Powerbook.
I've never had the chance to use one of the current G3 iBooks, but I hear they are pretty speedy, unless you're doing lots of CPU intensive stuff (rendering etc...).
I couldn't make a comparison to a PC laptop. My partner just got a top end vaio, and my 867Mhz G4 seems pretty comparable. I also run Virtual PC when I need to test stuff out on XP. Sluggish at best, but useable for testing web sites.
The only thing that is noticably slower in Internet Explorer. It renders slow compared to a PC.