I'm usually a bit skeptical when all the reviews are completely positive and I'm also a little paranoid about buying stuff online, but after doing a bit more research on the site itself, it did look pretty legit. The price was much cheaper than retail and you get the code almost instantly. If you want the game for a good price and you can't be bothered going to the shops, this is a great way to get it.
Oh and yeah the game is good but you should download the free starter edition and see for yourself before making the purchase if you're not sure. You also unlock the hero Artanis in Heroes of the Storm (a different game by Blizzard) as a free bonus when making this purchase and using the code.