It doesn't need to have every feature in the world as long as what is in place works nicely, support is decent, and the user base is sufficient so that upgrades will be forthcoming. This fits the bill in all areas.
Setup is easy.
I was able to prepare and maintain a user list separately, from my main site, without too much hassle. Message searching is very nice. The threading issue hasn't been a problem. A Product of Matt's Script Archive, Inc.