That is pretty unique. We usually die when crashing unto other ships, which is the case here when we are not bulling. It is cool when you line 'em up and go for multi kill!
If I may suggest, consider adding a meter to show the 'bull' cool-down, it helps with strategy, and I didn't notice if there was a mass-kill bonus, which would also be nice with some effect (sound and visual). I also have some more, if you'd like :-)
Finally, I would have liked the controls on the left like the handheld...