Rock is far faster than Cydia, which is as slow as hell and needs to download and reload a bunch of junk every time you open it. Its interface is far more intuitive; it's easier to use and almost resembles the App Store, while Cydia is designed for people who know what the hell they're doing. In other words, there's less of a learning curve.

As far as I'm aware, security and software problems are the main reason Rock hasn't overtaken Cydia. There have been various glitches and problems with Rock in the past. Notably, there was a glitch a couple months back that required people to restore their iPhone to factory settings in order to uninstall Rock. Not good stuff. However, my first impression of Rock has been good.