I use a Mac, so I haven't had that much experience with FruityLoops - but from what I've seen it's great for composing. It's basically set out like a 16step sequencer that you can assign individual sounds or loops to. It looks real easy and quick to use. However whilst it is good for composing, you will probably want some decent audio sequencing software to bounce your tracks into for your final mix and master.
Reason has a cool interface in that it represents a hardware setup. however I'm not sure how easy it is to upload your own samples into it. I know the redrum sequencer loads .wav files, but I think thats all. Fruityloops looks a whole loy simpler.
Does anyone know of any programs similair to Fruityloops for Mac, coz it looks really simple and quick to use as a compositional tool. (Not that it will ever replace my MPC1k)