Its just OK.
Try Kim Aldis Swuff first, it doesn't have the great compression engine that swift has but swifts was kind of dissapointing anyway based on all the hype.
I also expected far more integration with XSI rather than just a very basic exporter with little control of the resulting flash edges shapes etc.
As an aside perhasp I'm asking too much here but how about some actionscript or expressions conversion in XSI to flash and perhaps specialized for swift shaders and shader previews.
Anyway the speed is not as great as is made out.
Perhaps try swuff here first.
http://www.aldis.org.uk/3DResources/Scripts.htmhttp://www.aldis.org.uk/3DResources/Scripts.htm
edit:
oh yeah, I own swift for XSI