I would say it is. They are actually good for modern applications (provided they support powerpc) except for, as said, flash content.
That said listening to itunes and browsing macrumors it feels largely the same as my Mac Mini.
Just make sure to max the ram and put leopard on that baby and you're good to go. It was my main machine for a LONG time (5 years or so of ownership, about 3 where it was the only machine I had). I still have it, but my new Mac Mini is now my main machine.
Also, if it doesn't have dents or scratches, snatch it up, its unbelievable how horribly people treat their powerbooks by looking at ebay.
So point of it all. Pick it up 🙂.