He might have been a bust for the Bears, but he was actually solid for the Bengals. I think it's wise to let him go now, he's a RB, they are disposable objects in today's NFL. But benson actually had a nice run in Cincy when you look back at it.
2008 (joined team during the season) - 747 yds - 2 TD
2009 (13 games) - 1251 yds 6 TD - Bengals made the playoffs
2010 - 1,111 yds - 7 TD - 1 rec TD
2011 - 1,067 yds - 6 TD - Bengals made playoffs
so in 4 years (3 full seasons with the team) they made the playoffs twice, in 2009, the offense it was largely on the back of his running. He also ran for 1000 yds 3 straight years.
He never got off the ground in Chicago, but he actually turned out to be a nice grinder in Cincy.