kururunfa - 10:05 pm on Feb 21, 2012 (gmt 0)

I ran eight sites on a MU install for a long with no serious issues (a couple of plugins had trouble, but replacements were easily found).

The only thing I'd say to watch out for is that MU can be a bit of a resource hog and some shared hosts don't like it. In my case the sites were low/moderate traffic, on a VPS.