Well, after seeing the Eurogamers review of the installs, it seem to depend on the game. So are a LOT helped and some aren't helped at all. I think Microsoft should make a list of wich games benefit from it, since games like Halo 3 are significantly slower with an installation, while games like the orange box are lighting speed with an install..

http://www.eurogamer.net/article.php?article_id=290745&page=1 

ps: It's still (alive) fun to note that M$ said for years that the ps3 was lame of letting people install games (in general on the ps3, you choose if you want to install the game, mandatory installs are pretty rare) and that they're not like this... well, it seems they oriented their update on that :S