My screensaver is 90% visible and 10% transparent (as all other apps are by default). I hate it when a window pops up in the background behind another one and you can't see it, and I hate even more when a window pops up in front of the window you're working in.
After all, they are windows, they're supposed to be transparent.
Anyway, transparent screensaver may look cool, and may not, depends from one to another. From my experience "BSOD"
look awesome when transparent, also flying toasters and such. An images slideshow screensaver doesn't look so good when transparent though.
Edit:@rylius: I never liked the sweeper, I prefer thinking of the scene as a pool of water with raindrops on it.