So far this season on offense, the Bengals have a -3.1% DVOA (that's below average compared to all 32 teams in the league with 0% being average). The passing offense ranks 22nd with a -6.9% DVOA; the rushing offense is above average with a 1.9% DVOA -- ranks 9th.
For comparison sake, last season the Bengals offense had a 19.6% DVOA -- (27.0% passing, 11.1% rushing).
This season, the Bengals offense has scored 111 points (10th in the NFL). They are on pace to score 355 points falling 66 points shy of last season's total (421); that's 4.1 points less per game.
Palmer's 8.4 DPAR ranks 18th and his DVOA is below league average. Last season, Palmer sported a 108.3 DPAR and 34.5% DVOA -- both second to Peyton Manning. Rudi Johnson, like Palmer, is around average with a 7.4 DPAR and 3.5% DVOA. Rudi, in 2005, was in the top three in both DPAR (50.9) and DVOA (22.7%). You've noticed Chad Johnson's numbers are down this season -- ummmm, duh!. He too is playing around average (with all things calculated as equal) with a 2.7 DPAR and -6.1% DVOA. Chris Henry is hovering around the same percentages. T.J. Houshmandzadeh is by far the team's best receiver this season. His 9.0 DPAR and a 29.8% DVOA (7th in the league) is the team's best.
So yes, the Bengals offense is off this season. You can blame play calling and injury all you'd like, but comparing these guy's success rate on an individual basis shows their production is down.
Defense-adjusted Points Above Replacement. This number represents the total number of points scored due to plays where this QB passed or carried the ball, compared to a replacement-level QB in the same game situations.
DVOA breaks down every single play of the NFL season to see how much success offensive players achieved in each specific situation compared to the league average in that situation, adjusted for the strength of the opponent.
Explanation can be found here.