The Hangover Part 2 of course contains the same cast, plus a clever monkey and a few other characters to accompany the 'new' storyline. This theme of similarity follows throughout the film.
The storyline practically follows the same twist and turns from Hangover 1. Which at first I thought was a good idea, as it reinforces the comical element of the 'worst hangover ever!' as they have to run through the exact same situations. This can be looked upon in reality, we always have hangovers which you feel are just always the same.
However, this idea of similarity or in fact identical story brings the film down. You can't help but think, you could have just purchased the first one and saved yourself a fair bit of money from the overly priced cinema tickets. Part 2's only difference is the small selection of extra content which could have been used in the first one to improve the story line and grab more gags. And of course the location setting for the film is different, Bangkok.
Ultimately if Part 1 and 2 would have been merged together into perhaps a 2 hour film, you would witness a genius piece of comedy. Which would have you laughing from start to finish.
Having said that, Part 2 is still good and has its laugh out loud moments. I just wished they changed the idea of hanging exactly the same hangover a small bit. It simply shows laziness.