I like shootemups a lot, but Beat Hazard turned out to not be the game for me. It has a pretty solid sense of style and presentation, but beyond that, I think it gets a lot wrong and has little to encourage me to continue playing it.
If you'll be playing with keyboard and mouse, I immediately recommend against Beat Hazard. While I'm told that the gamepad controls are like what you'd expect if you've played Robotron 2084, Smash TV, or Geometry Wars, the keyboard and mouse controls are an awkward design where WASD controls your ship and your mouse controls a cursor which your ship aims at. All shots you fire will hit wherever the cursor was when they were fired, so if the cursor is below your ship and you move your ship right, you'll go from shooting straight down to shooting down-left. This setup means that you're controlling the direction in which you're firing with BOTH keyboard and mouse, while with a gamepad, you'd only be doing this with one analog stick (the right one, used for aiming), as it should be. This one simple issue does a lot to drag down the experience, as it muddles the controls and makes it too difficult to hit any ship you haven't explicitly put your cursor on as you're trying to dodge everything that's flying at you.
The presentation is slick and the colors sure do look good on my Trinitron tube, but the game does way too much flickering that makes me feel a little uncomfortable as I play it, despite lacking any sort of epilepsy problems. Furthermore, the enemies' shots can be rather hard to see against the exploding colors that make up the background. Also, for a game supposedly based on your music, I can't really tell what it's using my music for. Yes, my music plays during the stages, but it never feels like the game is really driven by what's playing, unlike games like Audiosurf, where there is a clear correlation.
Finally, as a shootemup, I feel this game really falls short. It doesn't feel at all like most games of the genre, with enemies that take way too many shots and don't shoot back nearly enough. It also has not nearly enough enemy variety; expect to mostly see the same ship over and over for each of the 3 classes of small, medium, and large ships. I love the variety of enemies in Geometry Wars and similar games, but this definitely does not have that. Additional variety would contribute not only to making each stage more enjoyable, but improving the game's longevity, as well.
There may be something to the game I'm not seeing; it's a pretty popular game, and if you're playing with a gamepad, you're already set to have a lot more fun with it than I did. That said, I don't have any reason to recommend buying the game.