For me NOTHING is off limits when it comes to comedy (or anything else for that matter, as long as it doesn't directly incite violence toward another person).
As someone else said, I find him about smug and self-congratulatory and therefore some of his stuff isn't all that funny, though he can bring some corkers at times.
I am bisexual and I don't think bisexuality (or LGBT in general) should be free from mockery or jokes. As long as they are clever and thoughtful in the execution then there isn't a problem for me. I am not a huge fan of gay = effeminate etc but I am also aware that stereotypes come from somewhere - they don't materialise for no reason.
I don't think I've ever been offended by ANY speech, let alone a comedian. I chuckled at "that" Katie Price/Harvey joke by Frankie Boyle, the 6 million Jews gag that Jimmy Carr did, THE Jade Goody and cancer joke and many others. Do I condone killing minority groups, want people to get cancer or think fat learning disabled children want to have sex with their parents? Of course not, that is ludicrous and that is the point. Comedians say the most ludicrous things, it is not meant to represent real life.
I think Boyle, Gervais, Carr et al are all characters anyway. if you listen to any of them (esp Boyle and Carr) they will very articulately explain why a joke is meant to be funny.