I've done them a few times. Just for the odd weekend in a caravan. They are usually for Fri to Monday or Mon-Thursday I think.
The first one you book is a bit of a gamble. You have to collect the newspaper tokens and apply by post, giving three different sites and dates I think (from memory). We had a chalet somewhere in devon and tbh it was absolutely fine, just a bit on the small side.
Once you've had one holiday, you get a priority booking code the next time and can book online. So that if you look at other websites (Money Supermarket and Netmums) people list the codewords so you don't need to buy the paper. They also guess the last codeword so that you can apply before the last newspaper token comes out and get the place/dates you want.
We've stayed in caravans where we already had an idea about the site. We go for small sites with little or no entertainment.
It works out around £40 for the 4 at £9.50 and there's usually a cleaning charge of around £20. Some also have entertainment passes but I don't know if you have to buy these or whether it's optional.
There are more dates out of school holidays than in them. But I think it's cheap for a three night caravan or chalet. We tend to go Friday night and even if we've not got the Monday off, we've at least got the caravan all day on the Sunday until we feel like packing up.
HTH