Exerion is an (almost) Old-School vert shooter that makes copious use of momentum, inertia and all the other things you hated calculating in Physics class.
It's a very well programmed game. The enemies are cool and interesting, the graphics are nice and unique and the controls of your ship make the gameplay tough. Graphically, the backgrounds, hills, and parallax scrolling add tremendously to the 3D effect and the feeling of momentum and inertia. in fact, the terrain movement/scrolling came within a whisker of giving me Ecco the Dolphin Syndrome. I felt just the slightest bit wonky - but no worse.
There are two buttons in the game. Button 1 is your normal (double) shot. How fast you can shoot is limited. Button 2 is rapid-fire (single shot), but the number of uses you have is finite. There is an indicator at the bottom named "Charge" that tells you how many more of the rapid-fire shots you have left. Luckily, killing enemies is what increases this number, so it goes up & down a lot. I suggest you use button 2 a lot at the beginning. You'll need it to survive the first few enemy formations, and by killing them, you will keep it largely replenished. Later, tougher and bigger enemies make it more important to use these judiciously.
The enemies come in small groups/formations and they look nice and are hard to pin down. More importantly, you quickly get to face "Phoenix" like bird-enemies that fly and react similarly to those in that classic game (Phoenix). And let's face it, the more a game is Phoenix-like (Phoenician?) - the better.
There are some other games that employed this early parallax scrolling to give this effect, but still, Exerion is a somewhat unique experience. Great enemies, strong enemy A.I. and the crazy movement make it buttloads of fun - just don't play right after you've eaten pea soup.