I like the core(ha!) concept, but I feel like controlling the orb is kind of difficult. I suppose that's the point, but it makes things that should be simple a little more difficult than they should be. I don't know that I really have anything more constructive to add to that, just that at times I didn't care for it.
On the other hand, the sprite work is great, and the music has this chaotic feel to it that I really dig. Overall It's pretty good, I just had some difficulty controlling it.