It's up to you. I really don't think they'll check your age. They'll ask for your passport and make a copy of it, that's about it. It's not really an inconvience if you arrive during the day (if you arrive at night, that's a different story). Around 4PM everyday, I know I'll be at Oasis which is like a 5 minute walk back to the Omni. Andrew is also really reliable and will be at Oasis at 4pm everyday. So is Chad if he goes for the whole week. Worst worst case, book through a site that tells you the minimum age is 18 (such as expedia) so if they really won't let you check in, you can tell expedia to move you to a comparable hotel for free because it's their fault. Again, I really don't think it'll come down to that. ME is really nice. It's nicer than Omni. I'd give Omni a 4 star and ME a 5 star. It reminds me of a really trendy and sophistic W hotel. I REALLY like the atmosphere there BUT most of the rooms (except the suites which Real World people stayed in) don't have a balcony. I'm pretty sure it's one of the few hotels in Cancun that doesn't have balcony in every room. If ME is in any other city, it'd be one of my favorite hotels (and I've probably stayed in 100+ hotels all over the world) but no balcony in Cancun is a big turn off for me.
Being responsible person and knowing how to party are not mutually exclusive. The difference is that a responsible person knows where and when to be crazy and silly.
