I got the SB600 for my birthday in January, and I'm obsessed with it. It's awesome for indoor shooting in really low light situations (like when even my f/1.8 can't go handheld). I use it a lot now to photograph my hybrid stuff, since I usually don't finish those til it's dark out, but I want nice lighting.
Anyway, I went with the 600 over the 400 because you can rotate the head on the 600. I keep mine pointed straight up 99% of the time. Definitely invest in some rechargable batteries though, it goes through them pretty quickly (or at least mine does). If you want to use it for portraits, it'll be wise to invest in a diffuser for it.