Well Ken already added some great tips about Superfly render settings. I understand what you mean about not being able to use the computer for other things when using the GPU rendering. But I think this is more a video card thing. My understanding is that unlike a CPU, a GPU can't say, oh, here's another thing I need to do, let me take a few resources and do that. So basically when you are doing a GPU render, it can't just easily draw other windows on other screens.
That being said, I don't often find that my renders take very long. I've learned through lots of trial and error where to tweak my settings and sometimes it might take half an hour, but rarely more than that, unless I've got a huge number of things in my scene. That is one other things about Superfly. It figures tries to determine if everything in the scene is effecting the render, not just what is obviously in the camera view. Actually my avatar took a long time to render, because even thought you can't tell, the thing behind her head is part of a dragon. It was originally meant to be a massive scene, and when I decided to do a close up, it still wanted to consider everything else in the scene.
