I was weary as well, but figured I'd try it out. It worked and I had no problems. When you purchase the game (it even gives you the option of PayPal or Amazon if wanted, along with any normal debit or credit card of course) it gives you the keys. From there, open up Steam and in the bottom left corner you can choose to redeem a key, and that's where you copy in the code. My purchase came with an extra game, too.
Done. Thanks for the link.