I know this has been battered around a bit but I am looking at the same thing, I found a similar service offering to let you have at their gre practice test until you get it right, OR they will give you a refund if you don't like anything about the test.
My GRE could use some improvement, and few things in education are guaranteed so I'm tempted to jump all over it.
I didn't read through all the flaming and pointless discussions, but did the gre prep work well Dr.Faustus?
Thanks in advance for your time.