It was a huge money-maker for a gym I used to work at, they did them weekly. It probably helped that it was at a larger sports complex so parents could schedule tennis matches or fitness classes for that time each week, have their kids doing something fun, and not have to worry about getting stuck in traffic or back late to get their kid. It was basically an open gym, sometimes the coaches would set up games or an obstacle course, and a slice of pizza was provided. I can't remember cost for sure, but I think $10-15/child for 3 hours. The gym was also right in town and very convenient to lots of restaurants, a movie theater, and the mall which I imagine contributed to its success.
I've suggested it for my current gym, but we are in a much more rural area with the next town about 20-25 minutes away. So if parents want to do something beyond getting dinner at a little local place or hanging out at home for 3 hours, they might have a bit of a time crunch to get back on time. Though I think lots of folks would still eat it up. It's comparable in price to a sitter and the kid is doing something fun/different and will likely be exhausted when parents come back for them. It's also pretty minimal in terms of planning & set-up, and if you do it just a few times a year I think the novelty factor would be huge. I think, like anything, it's about how you sell it, but I do know they can be hugely successful.
