Sound. Pulse. Radiance.
It is many things, from Mizuguchi's first offering of synaesthesia to one of the few rail-shooters to come out in recent times (fittingly, it seems, done by some of the creative minds at Team Andromeda). It was as much a music game without being quite a rythm game, as it was a rail-shooter without being quite a blast-a-thon.
A game of this type need not establish a very convoluted backstory to justify the pulsating music-based shooting. It is very sparse and very simple, yet archaic and ambiguous enough to keep a sense of enigmatic driving purpose. It may not make any sense, nor can anybody argue that it's visual representations of a digital landscape any more convincing than Tron's, but it all serves to reinforce the trance and the vibe the game exudes.
To experience Rez through gameplay is to experience a melding of several different aspects of a game all at once. Normally, we would consider Sound, Visuals, and Gameplay separately, and weigh their contributions to the whole of the game. In Rez's case, they are intrinsically one entity. Everything has a place and a purpose. The visual is choreographed to the beat. The beat is produced by an action. And every action creates a visual.
The game focuses on eliminating hostiles to a rythm, but the shooting doesn't get to the hardcore levels of Panzer Dragoon. Full 3D aiming is only granted in boss fights, but the rest of the game is a funnel that has you zapping targets through scripted and linear visual panning. Rez is truly an experience for the ears and eyes, but the game doesn't offer much of anything else beyond it's core shooting. In essence, it is a very trippy amalgamation of sounds and lights, with little much else apart from the shooting. It's a straightforward and simple burst of a synaesthetic barrage.
Unfortunately, not many people will "get" this game. It is definitely not for everybody. It may take an acquired taste to fully appreciate a rail-shooter, espescially one as deviant as Rez. Though it is a simple and straightforward game with not much else to it, it is still recommendable for people who are willing to receive a very different and very refreshing assault on their senses. Welcome to synaesthesia.