I took my children on Thursday - it's really good and I'd recommend it.
my kids are 6 and 11, and both enjoyed it.
It doesn't take ages to walk round, but they do have costumes and pretty much EVERY monster from series 1, 2 and 3.
You can take as many pics as you like, and it was VERY easy to just stand and pose for photo's without any pushing and shoving or hassle. Bearing in mind it's the school hols I was surprised it wasn't busier, although they do sell tickets in 30mins admission slots (but you can take as long as you like once inside) to break the crowds up.
I think the family ticket was �37.
not sure about parking - you may be able to park at earls Court (check with them) but I'm sure all the local roads have parking restrictions.
Go though - you'll enjoy it.