I have the Groovy Chick -- a recent purchase -- and I didn't even bother to try the bullet that came with it, I just pulled that out and replaced it with a Little Chroma. It transmitted the vibrations wonderfully. I plan to review it once I've figured out if a few other bullets work with it, and of course once I've actually taken the little paper bit out of the bullet that comes with it, because I suppose it wouldn't be a fair review otherwise.
They really are as weak as the bullets. The creators of rock chick do have a strongER bullet than most, but it's still just a bullet. When you consider that there's a single vibe trying to work for both ends, it doesn't wind up lackluster, imo