We've been using it for 3 months now in our team of roughly 45 people - 20 devs and 25 testers. This week we've had 1000th code review completed.
Here are a few stats, as of right now:
1008 change lists
17081 file versions
2863 review iterations
If you work at Microsoft and would like to try Malevich, I've set up a test change list here: http://sergeysomsg2/Malevich/default.aspx?cid=299. You can add yourself as a reviewer, leave comments in files, and submit review iterations.
If you're outside Microsoft and want to try it, the source and installation instructions are here: http://www.codeplex.com/Malevich.