Like night and day: everything feels so much snappier on XP than Vista. OpenOffice loads in half a second rather than ten. Thunderbird doesn't crash. Wifi connections stay connected. Locking and unlocking the screen happens instantly - no more long delays.
Downsides: I miss the way Alt-Tab on Vista displays thumbnails of all the windows, but TaskSwitchPowerToy.exe from MS at least lets me click on an icon. I still sometimes hit the 'Windows' key and start typing something to search for, before realizing I don't have a search input any more.