Good post from Sean Gallagher at Ars Technica, he noticed a few of the things that I thought were surprisingly unreported about XKEYSCORE. Basically the tool can tell you available systems to be hacked, find who was using encrypted VPN sessions, and grab other encrypted items for later decryption. Yikes.
Side Note: I cannot help but be impressed by how robust these systems are. Truly stuff of the movies, actually more powerful than many movies had imagined.