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The milkman conspiracy was a very fun level for me. The fun mostly comes from just screwing around like you do in the camp, trying wrong objects and getting caught on purpose just to hear all the questions the agents can ask (there are tons of those it's funny) you and so on.
I'll admit it's not for everyone tho, it's one of those level who show how much while being platformer, Psychonaut is a tim shaeffer creation and thus heavily influenced by advnture game.
As for the meat circus, I alway felt ambivalent about it. I think it's an awesomme level and I never understood all the hate for the rising water section, sure it's a step up in diffciulty but it's not that hard and it's it'd be be a challenge. The section with little Olie, however is just annoying and painfull. Boss fights we're disapointing.
it's an excellent game altogether. it's only default IMO is that it is a bit short and really doesn't have that many non tutorial actual levels.