Go! In the past they have been great. Don't expect to stay longer than an hour or so, but lines can be long (~20 minute) for the pin traders, buying the new pins, or photos with Mickey and the crew.
The cookies have been wonderful. I think I remember it was cider to drink, but adult stuff for purchase.
Go! You'll have a great time. Mixers are what you make of them; you will always find good things!
I have been to many of the Merry Mixers over the years and you will enjoy yourself. There is usually a character meet and greet so you daughters will enjoy their time there as well.