Mantis definitely wins on usability grounds over Bugzilla.
In particular, it is just a lot faster to log bugs on Mantis. Time to log bugs is a blocker for some people - I've heard it used as an excuse for not logging them, fixing them and pretending there was never a bug to fix (symptomatic of deeper team problems).
It wasn't until a client (currently using Basecamp, bleah!) canned the idea of Mantis because it wasn't pretty enough that I realised some people (as noted above) think it is ugly.
Compared to Bugzilla or another system we tried implementing, some weird European thing, Mantis is gorgeous.
I know Mantis scales well - a friend used it for the production of the movie Happy Feet. He customised it by adding one extra field to provide another level of categorisation.
