I have to disagree with MovieBob and, seemingly, every critic out there on this one.
I really wanted to like this film, but I was disappointed. It was exactly the same as the first one, and yes, I understand that's all part of the joke, but I didn't find it to be a funny one. The constant throwbacks to the first got tired (being exactly like its predecessor is why Hangover Part II sucked), the new additions were good but so infrequent next to the 'Hey, look! It's 21 Jump Street again! It's funny because it's actually a sequel to that film' jokes, and as weird as it sounds, the whole film was predictable. It's a comedy, so predictability shouldn't matter, but I knew what was going to happen from 5 minutes in. Why is that? Oh yeah, because 'Hey look! It's 21 Jump Street again! It's funny because it's actually a sequel to that film'.
It wasn't just me who found it mostly unfunny. The cinema screen I was in was over half-full, and there were only 3 instances that I remember where the majority of people watching the film properly laughed. There was the odd giggle every now and then, but it was mostly a disappointed silence. Don't think I've ever experienced that when seeing a comedy in the cinema, and I saw Meet the freaking Spartans!
Good parts? The already-mentioned-by-MovieBob Ice Cube meltdown, and the credits, which were creative and made a mockery of the film industry in a more original way than the film itself managed to do. Unfortunately, the repetition and unfunny jokes at the film's own expense just made me sad after the first 5 or so times.