I had some fun experience with GFW LIVE two days ago. Basically I wanted to play Flatout Ultimate Carnage on my laptop, and continue my progress.
So what I did was install GFW LIVE the new 3.3 version, moved the Flatout save file, and installed the game. When I started the game it asked me to login, so same account login was used.
And was greated with no save progress, was trying to force me start from start. First I thought maybe I missed some save file, and looked around the computer but there were no extra files.
Remmember that main PC is pretty much flawless and last time I installed OS from scratch was Vista, then just upgrade and everything runs like a dream, my point by saying that is that I never had to reinstall GFW Live before.
Anyway, then I realized that most GFW Live games, not only do they need same account name, but they also need same initial account login info. Basically first time you sign in, it creates bunch of tiny files (cache) in the xlive folder, then the save games somehow get tied those cache files, so if you reinstall and use the same account as before, but those initial cache files are gone(and new different files are created), your save games won't work.
Whoever designed that aspect of GFW Live was a complete brain dead moron.