I use it primarily on bass, but I've applied it to vocals, too, and also in parallel. I expect it'd be worth a try on just about anything you want to dirty up a bit. It was one of just two 32-bit plugs that made the cut when I moved to a 64-bit DAW (the other was Kjaerhus Classic Chorus).
Just trying 2.0 out now...lovin' it. Really makes a bass line cut through a dense mix. 14 Cabinet/Mic combinations. Woohoo! Cab "B" + U87 is my favorite so far for bass. No wait, it's Cab "B" with beta52 "bright". Either way, FF Saturn just went into permanent bypass on this track.
Can't wait to try it on some guitars.