Very rarely works when they have famous guest stars in sitcoms. Most of the Friends ones are pretty lame. Worst of all are the cameos in those cringe-inducing London episodes, especially Jennifer Saunders, June Whitfield and Tom Conti - the low point of Friends and of those three's careers. I can't look at them anymore - prostituting themselves for the dosh. The only guest star to come out of it with any dignity was Hugh Laurie, who was very funny in his brief appearance.
Of all the guest stars overall, my favourite was Christina Applegate as Amy(?). Brilliant performance and one of the best characters in the entire series (apart from Frank Jr). Truly hateful but very, very funny.