Since it's Gervais, it's brilliantly done, and it is wickedly, cruelly funny. So much so that the peak moments the humiliation is so sharp and painful that I have to pause it and take a break.
The stars that appear as distinctly unattractive versions of themselves are actually a pretty small part of each episode. The funniest so far is David Bowie, whose extemporaneous song about Andy is the most excruciating, funny moment.
What makes it brilliant is that Andy is a very sympathetic character. I'm rooting for him, even as things go horribly wrong, and then even more horribly wronger. This is perhaps the one way that the American "Office" is inferior to the UK version. No matter how odious Gervais makes his character, you can see that somewhere in there is a human being who just craves the approval and affection of those around him. Steve Carell's character is very funny, but he's more a collection of prejudices, fears, and resentments -- you don't see him as completely human.