I bought an iBook 1.33ghz with 512mb of ram for a friend's daughter. It was her first computer that was her own.
I started in Mac's myself with a Hackintosh and moved on to a 2010 Mac Mini, so seeing how the iBook ran was a mixed experience. The biggest problem was simply flash videos. Without tinkering, changing a setting here and there or using an app to play youtube through quicktime, flash videos are damn near impossible to enjoy playing. They'll play, but constantly buffering. But there are fixes for that. Download Mactubes or one of it's competitors, or search for "flash" in the Power PC thread here for tips. You should also download a G4 optimized Firefox 3.6.3 or Camino, they're specifically tuned to work faster on a G4.
I'd also say that the usual rule of the more ram, the better the performance.
But other than flash, the iBook plays DVD's fine on only 512mb, works great for surfing the web and word processing, etc.
If you're looking for a laptop and you're on a tight budget, an iBook is still a good choice. Just remember that there are many newer programs that will not run on a PPC cpu, but if you just want a basic setup, it's fine.