MT4 has been released under GPL, so I played with it for a bit last night. Except for a very very brief period of time, there has always been a free (beer) and somewhat limited but very usable for single blogger version of MT. Now GPLed MT4 is fully functional multi-user multi-blog.
MT has a few advantages over WordPress. It supports a few database backends (MySQL, PostgreSQL, SQLite) while WordPress only supports MySQL. MT creates static files for blog sites so the performance of the blog sites are better (no need to hit Apache/PHP upon every request), especially for popular sites. Multi-user multi-blog means you only need one MT installation for a bunch of folks each writing their own blogs or collaborating on multi-author blogs, which is really neat.
It is not w/o problems thou. No. 1 complaint I would expect is the lack of a Chinese language UI (deal-breaker for one of the projects I know about), although it handles UTF8 (for CJK) faultlessly. The developer community is also smaller than say WordPress or Drupal. I also prefer the MT3 UI.
Really slick multi-user multi-blog is the killer feature. I have no such need, and WP does the job for me and my users more than adaquately.