Currently using Ableton Live, found this a really nice program to use. Didn't take long to learn and everything seems really logical to do, if you know what I mean. I used to use Fruity Loops about a year and ago and then moved to Reason but lack of VST's is a serious disadvantage.
I had a play around with Cubase SX for a while but found the interface horrid, also thought it took ages to do fairly simple actions. Maybe I was missing the point?
Heard good things about Logic, perhaps if I ever buy a Mac I'll look into it.