We went to one last May, it rained, a lot but we got a couple of sunny days, we had a 'frills tent' in Devon.
We're not experienced campers at all, we were family camping virgins before we went, it was me and DP and our 3 boys (10,8,2)
. The tents are waterproof, we had a huge storm and not once did the elements get in the tent.
. It was freezing so the log stove was a god send, but we did spend an awful lot in extra logs, kindling, matches, lamp oil, wicks and candles.
. No hot running water, it's all from the kettle boiled on the stove, we took to keeping a giant kettle topped up all day boiling either on the log stove or the outside Fire pit so we could always have coffee, washing up water etc.
. Yes, they have the option of buying meal kits and hampers, they're expensive, it's not essential though, we did get one for a beef stew to cook in the couldren over the outside fire and we got a breakfast one, we justified it purely because we'd had s tough year and we were happy to pay for the luxury option as we knew it would be our only break.
. The kids adopted a chicken each and the coop was outside the tent, it cost £10 for the week and they enjoyed it enough that 2 weeks post holiday we had 4 chickens living in the garden 
. There is an honesty shop, again it's expensive but you'll need it for logs and you collect ice packs for the fridge box in the tent.
If you order anything they'll leave it in there for you with a clipboard that you fill in to keep a tally of your purchases during your stay.
. I'd take extra bedding, it's cold (although I find know about France compared to May in the uk) if you can get lamp oil, wicks, candles/torches then take them in bulk.
We loved having a tiny portable radio in the evening time, it was so quiet.
We took bats/balls/activity books for the kids, they used them all as well as exploring, touring the farm and assisting the farmer at the pizza oven.
The tent itself is stunning, very quaint and idealic, the fine details are lovely, hearts carved in the doors, the bed in the cupboard, the candle chandelier over the big dining table etc.
We loved it and did think it was worth every penny, I guess it depends on what type of holiday you like.
It really helped though that our hosts were so friendly and accommodating, they looked after us at all times and made the site feel homely despite still working hard to look after the cattle.
The contrast of things to do for the kids (away from screens!) and the evening hot tub/hammock/lazy relaxing with just DP was perfect for us.