I recently saw the new Chris Rock special on HBO, "Kill the Messenger," and Mr. Rock's stand-up is still very, very strong. The material was great - not significantly less or more great that his past few specials, but the format of this one was a bit different. This special took three different performances - one in New York, one in London and one in Johannesburg - and edited them together. I'm pretty sure it was all the same material on three separate nights, but spliced together. And this wasn't something that they were trying to hide. He was wearing different outfits on each night and addressed the specific crowds during certain bits. Overall, I'm inclined to think this wasn't a wise conceptual move.
In stand-up comedy a fair amount of effort goes into making the material seem spontaneous. Although this isn't obviously the case, the delivery shouldn't make the material seem "written." In reality, a comic can (and should) work on new material in a set also containing old material, or go off on unplanned tangents. Ideally, the audience shouldn't be able to tell what was unique to that set and what has been on a comics set list for months.
The format of "Kill the Messenger" works against that. It reminds the viewer of the mechanics behind tricking an audience into forgetting that the performer says this stuff exactly the same way night after night.
The expectations that a fan has of a comedian have always amused me. While most fans understand that comedians don't come up with material on the spot and say things that just come to them onstage, they do tend to feel that if they see you live, the material should be all new the next time you come into town. It's quite common to overhear people leaving a comedy club with a tone of disappointment in their voice as they say "I heard a couple of those jokes before."
This is in stark contrast to the world of music where fans exit in rage if the band hasn't played every single one of their greatest hits. Moreover, many fans only want to hear the hits. I was recently reading on a message board that at recent concerts of a very popular band, people had brought banners reading "No New Shit."