I haven't tried Mu as yet.. however I have helped a few friends set up wordpress.com sites on their set up it's pretty limited.. and I didn't like the way it integrated using their own domain name... it just basically forwarded or framed the site, so no real
SEO benefit to my clients using Mu that way...
I do NOT know though if that's just how wordpress has it configured on their end or possibly a server thing.
I have in around 10 sites now using WP to power them, or part of them (some are actual blogs on the back of another site lol)...
I appreciate the input though, thank you!