I saw a 1.5GHz in the Apple store today, and in the little bit of time I spent on it, it only seemed a little faster than my 1Ghz TI. Of course the machine was the 4200rpm/64mb vram version, and it's hard to gauge performance on a machine in a store.
When I've messed around with the G5s, however, there is a very noticable performance increase.