I haven't been myself, but lots of friends and family (who are Greek) have and they love Mykonos. Apparently there's the ultra-hip, very cosmopolitan and very expensive side and there's the gorgeous, laid-back, beach-cobble street-taverna side. So a bit of everything really.
I'm sure someone who has been will be on shortly to provide details.
I was looking into going to Mykonos this year but all the accomodation looked very expensive. The flights were cheap but the apartments were really dear so we didn't go. It does look lovely though, we would have chosen the quiet side of the island.