This is an indispensible plugin. Thanks for writing it/taking it on.
Couple of small issue/requirement:
1) it should be possible to edit a users group memberships from within admin, users, edit user (profile).
at presnt you can only asign/edoit group assignments via the bulk actions feature on admin, useres, list users
2) colour coding of member groups (assigments) - v. useful on admin, users, list users view
the plugin 'user goups' - though not covering as much functinal ground as yours, does have a couple of nice features - namely the above, BUT unfotunately doesnt play nice with your's (i think you maybe use different custom taxonomies perhaps?)
it would seem like a good idea to maybe merge plugins or at least adopt some of those features on yours. otherwise i'll need to do achieve myself with some sort of plugin/theme hack.
any plans to introduce such features?