It's not terribly great when you have large files in the system. You end up with a huge repository on disk as those files change. But more importantly, you can't do a partial checkout of a particular path. I think I read that that'll be coming to git, which would be fantastic.
Actually, you can, with sparse checkouts. They've been in since git 1.7, if I remember correctly. It's not very user friendly yet, and you do retain the entire path from the repository root. See for example here: http://vmiklos.hu/blog/sparse-checkout-example-in-git-1-7
