Clerks II is a funny comedy. The ending sucks and it's pretty hammy, though not nearly as awful as say, the shitty ending to Chasing Amy. That said the movie is hilarious.
Clerks, the original, is an American cultural milestone in film. Anyone that has worked retail can relate to the movie at a deeper level than Clerks II. It's a definite Everyman movie. Clerks II is about the characters, Clerks is about the experience. Nearly everyone has been in a situation from Clerks, whereas Clerks II is about the characters that have stayed in such jobs far longer than people typically have the ambition to.