I've been to a three and am going to another one in a few weeks hopefully. Of those three 1 was superb, one was pretty good and the other was a bit shit.
Couple of things:
1) DON'T get pissed. If people start getting drunk (and if you're nervous or don't know people well the temptation is strong) it all falls apart very quickly and becomes shit.
2) PREPARE! If everyone makes a bit of effort and dresses up well and practices any silly accents/knows their role etc then they just rock. It only takes 1 or 2 people to not bother and it has a knock-on effect and turns shit.
3) Don't worry about looking stupid. If everyone does it properly then nobody feels silly and the whole things becomes top fun. Same as numbers 1 and 2 - it only takes a couple of people to fuck it up.
Look forward to it, make an effort, be prepared to actually roleplay and don't get sloshed and it'll be a great laugh, trust me