If you enjoy arcade beat-em-ups Alien Storm delivers an enjoyable variation of the same and more importantly aliens.
However, where Alien Storm differs is its wider variety of gameplay modes which keep things relatively fresh and fun. It has an on-rails Lethal Enforcers-style shooter mode which is quite fun [hold down rapid fire and watch the carnage]. The relative ease of these missions and the mowing down of hordes of enemies present a light diversion from the main game. In addition, there's a pseudo-3D run and shoot mode replete with cheesy mid-air jumps.
In terms of presentation and controls, Alien Storm is quite satisfactory. The graphics and animations are good and the controls do the job well. By modern standards Alien Storm isn't amazing. In fact, it's decidedly trashy and B-grade but that doesn't mean the game isn't well-crafted or fun. Worth checking out for the Genesis or Virtual Console.