I used to work at one so I am all for them. My dogs went to work with me everyday. The one I worked at as a teen I had am older golden at the time and she did not play with the other dogs but she loved to hang out and she was an excellant teacher for the younger dogs. There are of course cons. Not all all dogs get along (they are like people and won't like everyone they meet) and sometimes fights can happen but if it is a good daycare they will get to know the dogs and will know which dogs play well together and which ones need to be in differant groups. When you have a group of dogs together you also risk them coming in comtact with dogs that give them things like kennal cough or whatever. I personally think that the socialization they get is a huge positive.
I don't, but I've had friends who take their dog and they LOVE it! Their dog was a rescue (1-2 years old), really energetic, but very shy/nervous. He really blossomed at doggie day care. I think getting all the excess energy out playing with other dogs helped a lot.
I think it really depends on the daycare, though, if you get your money's worth.