Yes I have bought tickets from them. I found that they had the best price for the tickets I was after. The tickets arrived very quickly and I was very relieved to see that they were actually tickets and not vouchers that I would have to exchange once we got to the parks.
So based on my experience I would certainly recommened them.
After I read Simon's post I decided to give them a try and ordered my tickets on Friday. Saved myself around £35! Hopefully they will arrive with no problems.