For our New Year's ('11-'12) cruise, which we booked in July 2011, the TA we booked with on Cruise Compete gave us 50% more OBC per room than anyone else. To make it better, there was some kind of issue with our OBC. After sorting it out at the Customer Service Desk, it turned out we ended up with roughly an extra $200. :)
If you can get a significant concrete benefit, then this makes going through a TA worthwhile. Most of the big US cruiselines have banned that practice now, unfortunately.
