I was glad of the frequency of open houses, though our situation might not be so common. When we went on our cruise a couple of weeks back my youngest, Alexa was just over a week of being 3 (she turned 3 properly 2 days after we got home). There is only a year and 9 months between Alexa and her youngest brother Owen, and those two are nearly as close as twins. Leaving Alexa at nursery wouldn't have worked because she really misses playing with Owen (he's only just fully started school so they are used to being together) and without the open houses we wouldn't have had such good family play time together.
That said before the cruise I thought the boys (4 and 8) would probably only spend the odd hour there because we're usually together as a family so much. After the first day, when I dragged them out, with them begging to stay longer at 10pm after a great magic show where they got a little rubber fish each (Bluebert) I realised I'd underestimated it, and they spent quite a bit of time there.
The thing that impressed me so much was that the Lab and the Club both have some iPad games in, but whenever we went to get the kids, or went to an open house they were never on them, but always playing with the other kids.