After you learn the controls this is actually a quite addictive game (I changed the controls to the five lower buttons on the numpad). You start out with an arcade mode which goes from easy to frustrating hard. After playing arcade you'll unlock a new mode, and a new mode after playing this, and so on. There's actually quite a few game modes to unlock, where the last is the most interresting; namely the Challenge mode. Some of the achievements can only be obtained through this mode, and some are very different from the usual bunch, such as "get a better score than a randomly chosen person in a randomly chosen level".
Another fun feature that sets this game away from other side-scrolling flying shooters is the time-travel abilities. You can fast travel throw time, which gives you bonus score, or you can rewind time such that you can replay a few seconds (while your other timeline self's shots still count).
An interresting twist on a classic game genre, and I can only recommend it.