It's a lot faster than 6. I know that this isn't a very good measurement, but XP's System Properties reports 667 MHz on a G4 1.25 SP, vs 400 MHz with VPC 6. In general, it 'feels' faster too. Printing is much better, the printer driver is now installed as part of the Additions, and it just links through to the Mac driver. In other words, you no longer need to install a Windows driver for the printer.
So all in all, it's a good upgrade