If McGahee does play this year, and I think he will, the Bills are not expecting him to have most of the carries. Mostly they just want to have an indication of what they've got in McGahee. Look for him to average 5-10 carries a game once he gets going.
Henry looked out of sorts through most of the first quarter of the season, but seemed to have regained the knack vs the Bengals. No the Bengals are not the Buccaneers, but their front seven on defence isn't that bad. Remember too that Henry was hurting with torn rib cartilage. Look for him to continue improving.
It's not really that Bills fans are off the TH bandwagon, though Bills fans are like the fans of any other team (when a player is having a slump some fans will turn on him). It's just that when the Bills drafted McGahee, we figured ultimately there would not be room on the team for two starting running backs. The Bills did not draft McGahee to be a backup. I like Henry in spite of his slump, but eventually (2004?) he's going to end up on another team. That's just reality.