I can confirm that GFWL DLC works flawlessly. I bought all 5 Fallout3 DLC and never had a problem. GFWL also works very well. Only major issues are related to your ISP & or router NAT status & open ports. All can easily be adjusted & only need doing so once if your PC security/firewall/UAC settings stop the GFWL client from doing it automatically. I have been using GFWL since it launched in 2007 & never had any major issues. GFWL does not slow your PC down at all its just an urban myth you can even easily have it running in an invisible mode so you will never even know its working in the background like any other Windows service!!
MS have plenty of GFWL support staff now & even a very detailed website with most of the tech help you may ever require. 99.9% of all GFWL issues are user security related. If you live in an unsupported country you can even create a new gamertag & MS let you select any country to get around the 26 countries supported issue.
Most PC gamers hate GFWL because it stops them from easily pirating the games & getting the benefits of GFWL like leaderboards & achievements!
PM me if you need any specific help but this page & the links on the righthandside covers most issues anyway:
http://www.microsoft.com/games/en-US...s/default.aspx