It's pretty tough, actually. I had to play around with it for a bit before I really started picking it up, and even then I didn't come anywhere near the Game Center leaderboards. There are also achievements, and three modes as well. Reaper mode lets you build up a POW meter to destroy enemies with your ship, Onslaught mode pits you against waves of enemies on your own, and Scavenger is a sort of survival mode that tasks you with collecting protons in order to keep your life up. They all play pretty much the same (move away from enemies and towards protons), but the twists are good enough to put a new spin on gameplay.
Silverfish is a great twist on arcade action, and it's a steal at US$1.99 if you're into non-traditional shooters. That's an introductory price, so if you're going to pick it up, do so soon.