Just because this comes up when I'm trying to write posts I figure I should just share it with you. We have done this for several years (at least 5, but probably closer to 6) and we did a smaller version when we lived in Utah.
We have four families that participate. For a time we had five and at times that was a little much, but four seems to work out well. Basically what happens is that we rotate between families turns watching all four families children. For example on the first week it is our turn to watch all the kids while the other three couples go out on a date. Then the next three weeks we are kid free and have dating opportunities. Since we live where the commute is awful and husbands get home late we do our date night on Saturday night. It is four hours long. We have switched start and end times. When church started at 9:00 we went 5:00-9:00. The bad thing about this is that it is almost impossible to go to the movies. We had 6:00-10:00 at one time. It was always a late night for the kids (and sometimes for the ones watching them). Right now we go 5:30-9:30 and it seems to work well.
Some of the things that we worked out: If you can't go you don't. If it is your turn to babysit and you can't then you trade days with another family. If we all have something the same night (usually a church activity) we skip that week and continue the rotation. We always have a snack. At one time we talked about doing dinner, but we don't. Sometimes we will bring a Little Caesar's pizza home and feed everyone. We usually have to feed our kids a snack before bedtime (another reason I like the 9:30 end time over the 10:00) after we pick them up. When a family moves we ask another family to join. All the families currently involved have 3 kids. One family their oldest has never come and often my oldest goes with her friends and isn't there either.
It works out really well and gives us at least three dates a month. Often we end up at home catching up on shows and eating Chinese food, but we have that quality alone time. The kids love it because it gives them a time to be able to play with their friends. Our group is formed up of church friends and they often only get to see them at church (two of the families go to different schools). I highly recommend doing this if you have young kids.