This is not a scam. You do not have to pay. Check your email after the test is done. Your score will be there. Nothing to buy unless you want the full detailed report. Not only that, they won't sell your email to spam lists or anything.
A lot of the negative comments on this website stem from the misinformation that you have to "pay" for the service. This is not true. At the end of the test, you receive an option to pay for an extension of the service which provides you with a much greater detail on your IQ; once again, this is totally optional. When you type in your e-mail address, you receive a free report in your inbox telling you your IQ. You don't have to pay for anything. In addition, when you receive the e-mail, there's a link to remove yourself from their mailing list if you choose.
The site DOES give you your IQ for free. You won't see your IQ on the website unless you pay (i think. i never paid), but it does go to your email address.
I can't vouch for it's accuracy, but it did give me a result in line with my expectations and it is amusing :P