Rock is a more stable application than cydia, but it has its downfalls. Everyone enjoys cydia, so thats where all the REPO's and APT's are. Once Rock decides they are going to be completely free, im sure many more people will talk about it. A simple google search can help you out with more information.
I personally use both of them, cydia for most big applications, rock for fontswap, skins, and icons.. just because rock downloads in lists and i can queue up tons of them and download faaaaaaaaast.