It still uses Steam and you still get Steam Achievements. It just prompts to launch Impulse/Gamestop to install and then immediately launches Steam for you with a pre-populated Steam key. Very slick.
Plus you can use PowerUp Rewards if you have it to get credit for the purchase, or use any GS credit you may have.
Edited by ElektroDragon, 20 September 2012 - 10:50 PM.