I've signed up for it and got a few perks. None big enough to remember. But its basically your typical rewards plan, where you get a incentive to make Xbox or XBL marketplace related purchases.
If you are person that purchases DLC, games on demand, videos, music, XBL points alot. You will see good benefits from it.
If you are like me and only annually purchase COD map packs with MS points, I doubt you'll ever notice a benefit from using it.
They did have a special recently where if you purchased certain full featured games from XBL Marketplace.. you got like an 800pt bonus. Thats pretty awesome, especially considering some of the games were 50% off. Too bad COD4, MW2, or BLOPS1 wasnt on that list. I'd purchase those just to finally get rid of the stupid discs. Maybe next time!