It is very good, IMO, but it can be busy in the car parks. Once into the forest, it's pretty empty, though.
I think the charge for a day pass is £2. As it is private (Crown Estate) land, they need insurance etc and the fees cover that.
The "expert" section can be as expert as you want it to be. Some of the down hills are pretty daunting, with big drops and jumps, but I've taken my two kids there several times and we've never had a problem and always had fun.
It's a big area. Buy a map when you get there or, if you are tight-fisted (like me)
download it and print yourself a copy.
There's a snack bar / cafe there which isn't too pricey, so you shouldn't need to take refreshments other than liquids when you are out playing.