I use Fruity Loops Pro ($75 downloadable). The GUI is very straightforward with an "explorer" type window on the side to drag and drop samples on to individual channels. The samples that come with it pretty much blow donkeys, but I've downloaded samples from a variety of websites-there are tons out there with free downloads-and with a little eq and reverb, my ears can't tell the difference.
It even allows you to control the volume, panning, reverb, and delay of individual channels, and add VST plug-ins. It can also convert your loops to mp3 or wav files so you can drop the file into a track in your multitrack software. My only bitch is that it does not support tempo changes at this time. I emailed their forum on this shortcoming and they replied that this is a couple of releases down the road.
I don't know how this compares to some of the others out there, but it is very user-friendly and at least worth a look. http://www.fruityloops.com
