I don't really like the XSI compositor, but I don't really have a good reason... I think its because the flow is downward, and I'm still used to DFX+ which has the flow go to the right by default. I'm irrational like that
But yes.. XSI is a solid program. Over the past year I've migrated over to it from LW. I used LW for years, and now I can't figure out how I got by, because XSI just feels really natural to me... most of the time, anyway.