I was quite late to seeing the first Madagascar, and for some reason I ended up watching it - and had a bloody good time too, especially enjoying the scene-stealing side-characters of the two chimps and the gang of penguins (who are fortunately back more-than-before in this sequel).
Essentially it's more of the same from before, although with a plot that's a bit more scatter gun and far looser than in the first film - mainly due to splitting the four characters up so they can go off and join their respective species in the nature preserve in which the film takes place. Toss in another couple of side-plots, and you've suddenly got an awful lot of ground to cover in 90 minutes.
As a result you often feel like you're always missing every one of the characters, as they can often disappear for long chunks of time as we race around all the other sub-plots going on elsewhere.
It still works, but not as well in filmmaking terms as the first movie did (even if that one wasn't rocket science to begin with anyway). In the end though, you've got nice characters with morals to learn while giving the audience a considerable number of chuckles along the way - for me at least, most of said chuckles came from the penguin gang, who stole the entire movie.
More penguins please, huzzah! ... Or could that be a case of "too much of a good thing"?