Makes me laugh when people say stuff like "That's just the way he is" because it's not. The whole being controversial and offensive thing is relatively new in his career (bearing in mind he's being doing stand up for nearly 20 years) and is nothing more than a gravy-train that slowly losing it's wheels.
There's some clips of him on Youtube when he was in his early 20s, his stuff is as twee and mild as you can get, he even makes John Bishop look edgy. It was on Mock the Week where he (and the producers) developed this 'dark' thing, based on nothing more than the notion that every programme that features a panal must have a 'baddy' in it. On Pop Idol it was Simon Cowell, Frankie played the part on Mock the Week.
But it was a 'part' he was playing, until it became clear that he was one of, if not the most popular panelist on the show. He became big at that point which is where (IMO) it started going wrong because he started believing his own hype. Then after doing run-of-the-mill comedy routines for years and getting nothing he's suddenly getting all this publicity from being a bit controversial. But instead of realizing he had the balance right then he thought the more controversial he could be, the funnier and bigger success he could be.
There was a time where, IMO, he had it right he would make a joke and not worry if it was offensive as long as it was funny. Nowadays it seems he’s got this the wrong way around and doesn’t worry is a joke’s funny as long as it’s offensive.
I’ve seen Boyle on various things where he isn't prepared and doesn't have an auto-cue and to me he doesn't come across as a ‘real’ comedian, that being someone you can dump in any situation and they’re still funny and still quick witted.