It has a very specific methodolgy in how it works, and once figured out it works a treat. In saying that though, I'd leave it, and get yourself onto Discreet Combustion, one the best compositors in the world, many of the features comes straight from the higher end Discreet tools, such as the Keyer. The interface is miles ahead of Shake's too.
I take it by your nic. you use Lightwave? I used to use that for tv and film years ago, but moved on ot Softimage. The reason I mention this, is because the way that SI XSI works, and Maya to a degree, is very like Combustion, and Shake i nsome respects.