I've been wanting to enable comments on this site for a while. Unfortunately I was running Movable Type 2.64, i.e. a version from over two years ago that doesn't cope well with comment spam. The release of Movable Type 3.2 seemed like a good time to upgrade. Although Six Apart has a decent migration strategy in place, my old MT installation was pretty heavily customized (see early 2004 meta postings). I chose to do a "clean" installation, where I started with a new MT directory and migrated my plugins one-by-one, as need arose. I was able to get rid of a couple of them, since their features had been incorporated into MT itself.
I must say, for two years of development, MT 3.2 doesn't blow me away in comparison to the version I was using. More UI polish is definitely apparent, but there are still parts that feel clunky (e.g. the need for nearly identical comment entry forms on the entry and preview templates). The mt-search integration is still very crude (a separate template directory that's not accessible through the GUI). One of the use cases of MT (as opposed to LiveJournal or TypePad) is a techy user who uses their site as a knowledge repository (e.g. me) - in this case having decent search is very important.
Going back to what caused this upgrade, comments are now enabled. There's still some playing around to be done with CSS and I still haven't figured out what approach to spam I'll take (e.g. moderation vs. requiring TypeKey).
Update on 9/28/2005: Email addresses are no longer required. TypeKey authentication should now actually work (the trailing slash in blog URLs on the profile page is key).
As Evan mentions, Movable Type 3.2 supports OpenID authentication (albeit only as an extra). Supporting both TypeKey and OpenID makes the comment form a bit overwhelming. Since TypeKey is now an OpenID server, the latter should be enough, since it has a superset of the functionality. However, I think TypeKey still has more user awareness (and entering your TypeKey profile URL is not exactly user friendly), so for the time being I have both up.