Waffles has been going to daycare once a week for almost a year now - he loves it and we love it.
The daycare he goes to does mix the dogs, but they are not all together at the same time, there are several different areas. They do temperament testing, and they do not allow drop ins, plus you are expected to come at least once a week on the same day so the doggies make friends. We even get a little report card with the names of the dogs he played with each week.
Here's Waffles on his first day of daycare.