A parent's perspective - I'll try to explain
I don't want to patronise younger viewers: I was young once, I partied, I understand the appeal of drink / drugs / sex (that appeal doesn't just stop when you hit your 20s, you know). I do understand the appeal of this movie, and I have nothing against it for portraying those aspects of a wild party for young people. But I'll try to explain why, as a parent, I hated it.
Having a good time is great. It's cool, I still like to have a good time myself, despite my great age. But disrespecting other people's property and peace of mind while you are doing it is completely uncool, and that is what a) Costa and b) the party crashers did.
My kids are now in their late 20s. My son never held parties at our house, but my daughter did. Her friends were a considerate lot so we didn't have much to complain about, except for one occasion when one of the "guests" - a partycrasher - was conspicuously disrespectful of our home, our things, and other people at that party.
It is not a nice thing to come home to - sickmaking, in fact. I'm pretty sure that most of the people at the party had a good time (my daughter did, she was always the first one to get p*ssed and thereby fail to exercise any sort of control over the others). But you come home and find things broken, burnt, trashed, awash in alcohol and, as the one who works to pay for it all, it leaves a bad taste in your mouth.
Then along comes a film which, effectively, tells a world-wide audience of youngsters that I don't matter - as long as you're having a good time, trash want you want, it's OK!
And, depressingly, a lot of posters seem to think that that message - and it is the only message from this film - is perfectly fine (which means that they are OK with me not mattering - well, I'm sorry, but I do). The reason I hate this film is because I knew it would deliver that message and I knew that quite a lot of impressionable young people would eagerly seize upon it without ever showing any sign of empathy towards those at whose expense they are having fun, any sort of understanding WHY it so repellent if you are a parent, wage-earner, and homebuilder.
Yes, I know it's only a film. But if Costa was real and my son was friends with him, he would not be welcome in my home.