I have, a handful of times. The thing you have to add into the cost is the processing fees. If I remember correctly, you can get your tickets into your shopping cart and, before you pay, you can see what the actual total cost will be.
There have been times when it was the least expensive option for us and others where the fees pushed it over other options. Otherwise, though, they email you a voucher after you pay and you still have to take that to the ticket booth to exchange for an actual ticket. We've never had any issues the times that we've used them. I think last time we used them it was actually for Six Flags over Texas and we got a pretty good deal.