I've had great experiences at Mykonos, but also bad ones. The food is usually amazing, the service is generally excellent, and the belly dancer is always annoying.
Beware of the guests screaming Opa! and throwing a plate in the fireplace. It is loud and will scare the souvlaki right off your fork.
We went on my birthday last year and waited over an hour for food. Granted, it was a night where there was a special and two people at our table ordered it, but the fact it took them 40 minutes before updating us was too long. When our food arrived, only one of the six plates was as hot as we would expect -- the rest had obviously been waiting under an ineffective heat lamp.
Try the Mythos beer -- it's light, but with a lot of flavour.
When the kitchen is on, this place is fantastic. It seems to be family-run, and there is one waiter (he's probably about 22-25) who is very helpful. Again, avoid eye contact with the belly dancer.