I noticed this comment in the Rails trac timeline 
* DEPRECATED: Using additional attributes on has_and_belongs_to_many associations. Instead upgrade your association to be a real join model [DHH]
Couple that with the change to make habtm records with additional attributes
:read_only, and it looks like the time to switch to
has_many :through is now.
By the way, I've got a little list of articles I want to write showing various ways of using join models. If you have a request for something you want to see, leave a comment and I'll add it to the list.