Neat, I totally want a volca keys next. What parts/tracks have you used that one for? The fm is pretty nice. It's my first synth that isn't a plugin, and it's got a lot of capabilities that I haven't tapped yet. There's a lot of sounds in it including various cheesy ones.
Cheese is the best part ;) I've used the keys for textural stuff on a lot of tracks, and it features heavily into an electronic/new-wave project I'm working on now, Drive link to a rough mix of one of the songs here if you're interested. It's basically all the electronic sounds you hear on this, bass included. I've recently been messing around with the ring mod a lot, you get can some pretty gnarly sounds out of that thing. I would guess that there's not much that the keys can do that the Bass/FM couldn't, though I don't have access to either of those so I can't say for sure.