The Cyclonian Islands are absolutely amazing and Mykonos is one of them. Mykonos's Hora is full of fun bars and excellent restaurants. I hear that out of all the Greek isles, it's got the liveliest nightlife of all. I was there last summer and fell in love with it! Compared to Ibiza, I would say it is probably a bit more high-class, the average age a bit higher. As far as the crowd, it's mostly italians and greeks but it's a great atmosphere. The bars in Hora (they play excellent music, lots of house and techno) are a bit expensive so be prepared to spend your $$. If you're looking to party non stop, there is Paradise Island, it's the island's most famous beach, lots of good music too, and it's a younger crowd. Reserve in advance cuz the good accomodations book up fast and are hard to find. I would definitely not pass up the opportunity to go to Mykonos, it's a beautiful island!
While you're in the Aegean, be sure to get to Santorini, it's spectacular.