New Xbox Experience. We've tested it. So has Eurogamer. When it comes right down to it, the performance increase (if any) varies wildly on a game by game basis. If only there was some central source we could check to see how well a game runs after it's installed. If only there were some place where such information was arranged in an intuitive manner, alphabetically perhaps. Wait a minute! BingeGamer already put together a chart and it lists 360 games in alphabetical order! The chart also lists how big a footprint each game will leave on your HDD (Lost Odyssey is 23.2 gigs, yikes).
Not surprisingly, there are still plenty of games that haven't been tested, so be sure to drop a comment at BingeGamer if you have some data about a game that isn't listed yet. Big Bumpin', sadly, isn't supported, so don't bother with that one.