I've been running Vista SP1 for the past few days, and the system is quite stable. It was time to make Vista SP1 permanent on this machine. I had 45.6 GB free before running the above tool. Vsp1cln.exe is described as:
Vsp1cln.exe is an optional tool that you can run after you install SP1. This tool removes older versions of components that have been updated in SP1, which are stored during the installation in case you need to uninstall SP1 later. Saving these older components increases the amount of disk space that is used. Typically, you should run Vsp1cln.exe if you want to reclaim this disk space after applying SP1 and if you will not need to uninstall SP1. Note, however, that you cannot uninstall SP1 after you run this tool.
UAC asks for permission when running the tool, and a command prompt pops up asking if you want to make SP1 permanent. After the tool had run on my machine, disk space went from 45.6 GB to 47.3 GB. If you're sure you are going to be keeping SP1 this might be a worth while tool to run.