On August 22, Games for Windows Live will shut down their Marketplace, in accordance with the fact that Microsoft points are going away. This means that you will not be able to make any new purchases of games or DLC through the store, but you will be able to redownload existing purchases.
This does not mean that Games for Windows Live is completely shutting down. Titles like Fallout 3 and Dark Souls can still be launched and played. The activation servers should remain up for the time being. However, some titles, like those published by Microsoft, such as Fable 3 and Gears of War, will be unpurchasable from any store. Games that use GFWL but sell DLC through Steam should still be able to sell that DLC, however this is not 100% confirmed. If you’re unsure, buy now.
Some titles, such as Batman: Arkham City and Bioshock 2 appear to be getting reworked to use Steamworks instead of GFWL.
Games for Windows Live was maligned by many who had trouble installing or playing games using Microsoft’s middleware. I can count myself along those ranks, as I had some difficulty running Age of Empires Online, and had to read a developer’s post on a forum to find the fix. Some publishers, such as Capcom, have already switched away from GFWL to use Steamworks. With this, it appears as if other developers will have no choice but to also switch.