I would have unequivocally recommended Darkside if it cost any reasonable amount. However, considering you can download it for free, you have no excuse not to check out this most excellent arcade shooter.
Darkside has a lot of content and 3 game modes that offer several different ways to play. It's great for a quick dose of asteroid blasting and the mission mode provides a more structured take on the twin stick shooter.
In the end, Darkside may not turn the App Store on its head, but it's simple, good-looking, and proper fun, whether you give into paying or not. And sometimes, that's all you really need. Darkside doesn't innovate, but it does the genre some good.