Personally I think it would be really hard to manage. Say a guy built 50 great ponds, and only 2 of those people posted here. One good, and one bad. He would show only a 50% satisfaction rating, and that's not fair when in actuality he had a 98% satisfaction rating.
I think the pool of reviewer's would be to small to get an accurate percentage, and I doubt Bob would stake PB's reputation on a builder he had no personal experience with.
Just my 2 cents.
