This works perfectly. I wish I had found it a long time ago.
My sites have lots of custom user fields in preferences on the basis of which users add themselves to different usergroups.
The default usergroup promotions cron only runs once an hour. I can't tell you how many times members have PMd me to say that they changed their settings and nothing happened, even though the settings said "it can take up to a hour to see the change after changing this setting".
This addon fixes that. Members change their settings and see the effect immediately. Boom.
This is a great tweak for user promotions. We're using it for a User Preference checkbox and this makes the 'add to' and 'remove from' a User Group instant. xon makes stuff that should really be core XF but until that happens, I thanks xon for this contribution. :)