I've been using teh WebCTimport tool from Moodle.org with a pretty high degree of success, importing content, quizzes, discussion forums, and assignments all in one fell swoop. We've had a couple issues where quiz questions didn't properly import, and issues where HTM files in WebCT are not easy to edit in Moodle and need to be converted to Web page resources for easy editing. The other major issue is that assignments are all imported as "single file upload" assignments, where we would prefer them to be "advanced uploading of files"... Note that this tool has been recently updated and the tracker link above has some of the most recent info.

ps, for our course conversion, I set up a separate instance of Moodle on a cheap hosting server (hostmonster.com), loaded the webctimport tool, and use that instance only for converting courses.