XSI Is probably the fastest, most streamlined production tool for post production out there.
Bear in mind i said "PRODUCTION TOOL" - like maya and other "high end" applications its built for flexibility and robustness rather than fancy effects. It's used in film and broadcast (more broadcast at the moment) because you can really get in there and tweak the program around.
XSI has the advantage of all being in VBscript so you don't have to go and learn MEL for one.
Can't do a particle effect? just rap out 20 lines smack it into the scripted operator and off you go.
Want to draw out the same scene for later? Just open the command listener and its as simple as cut & Paste.
I haven't met one person yet who's used XSI (and by used i mean actually DONE something of consequence) and said it impeded their workflow.
the best place to learn about XSI is
www.xsibase.com - good luck with the course.
Bear in mind its geared towards being flexible though - its not as easy to do spaceships or whatnot as it is in MAX because as a whole the program is far more balanced.