They made another good move. At first it seems like it would hurt to lose their leading rusher. However, Football Outsiders says that there isn't much of a drop off from RB Thomas Jones to Cedric Benson. Plus Benson was a better receiver than Jones, although only 10 passes were attempted to Benson, so it isn't much of a sample size. Still Jones was a pretty bad receiver in 2006. Plus this move had been expected ever since Chicago drafted Benson in 2005. Instead they hold onto Jones for two prime seasons (2005 and 2006) and they are still able use him to trade him up 26 spots in the 2nd round.
Windy City Gridiron likes the move, although the commentators think the loss of Jones just to move up in the 2nd round isn't good value. I believe Jones is a free agent after 2007, although the Jets are signing him to a new contract as part of the trade, and Jones wouldn't resign with Chicago if Benson was around. Jones turns 29 in 2008 and signing him long term at that time would have been a risk. So to move up 26 spots, Chicago is giving up one prime season from Jones. That seems reasonable.
However now Chicago has to add some depth, but with RB Adrian Peterson around as a backup they can probably afford to draft a rookie and let him play 3rd string. Peterson played very little in 2006, but Football Outsiders says he played well when he had the chance in 2005.