Do the songs REALLY affaect gameplay or is this BS? WOuld like to hear what one of the earlier Black Sabbath songs with drop down guitar would do to this game...

*eyes bleeding* THIS GAME IS AWESOME *eyes bleeding*

I just bought this brainwashing software :D And enjoying it plently !

I don't know about enemy appearance and movement patterns, but shot power is affected by the intensity of the music. The game isn't geared towards calmer music. This can also cause problems when there is a low intensity section in a song, especially if a boss is being fought. Apparently enemy health is also affected by intensity, but fighting a boss on a classical song is suicide.
Hopefully some of this stuff is addressed in future patches.
On the steam forums, the developer is toying with the idea of adding AAC and M4A support (as a $1 DLC to cover licensing). He's also considering a low flashing option, as some have had trouble telling what's going on. Mind you, even with such an option, this game could still kill someone with epilepsy from a mile away.
Edit: Thread keeping track of suggestions, noting if they've been acknowledged, confirmed to be in the works, or have been implemented.

Yep, it's on my list....

God, playing most tracks by Muse is a sociopath-forming activity: all goes nice with rapid fire and lots of multipliers when, exactly at the point near the end hell of a lot of enemies appear and the tempo goes down and down and your rate of fire goes down and down and you're stuck with one bomb, dashing and dodging, and boom! You die, your last life gone.

This game is awesome.
Just unlocked Insane difficulty and it's, well, insane. I think Being swamped by seemingly endless waves of missile ships in the middle of Rise with only one life left was pretty traumatising.
Shame the game doesn't handle anything but intense songs very well, makes a decent portion of my library unplayable.
Also wished that there was a song by song scoring leaderboard like Audiosurf rather than just song length, seeing as getting to the top spot is more about the right song than skill it seems.

After reading a bit about it and seeing some shitty youtube video, I bought it. And I hate it. The issue is that it fucking slaughters the sound picture of my music, which is one of the best ways to piss me off in general. The browser in it sucks as well.

i got it too. at first i didn't know what to think, then got sucked in.
after two hours of playing i actually thought about playing Audiosurf, which i haven't played in a while. and i don't think i'll play it too often.
the game itself looks like a epileptic person's nightmare. if you know what i mean :)

I'm reviewing this game at the time, and it's really fun. Not suitable for playing for more than an hour at once, but fun!
Be warned: It can cause an epileptic attack. Even more than everyday game.

This will join Audiosurf as the games played at a party with a projector and low lights.

Beat Hazard now has a demo available, From what i can gather it's a 10 song trial.
So now you can five it a spin if you've been waiting for a demo

Pulled the trigger on buying this a couple of days ago. So far, so eye explodingly fun.

First game to give me a killer migraine after 20 minutes of play. It gets extra rep points.
I think it was sleep deprivation, I still stand by my description of it being epilepsy in binary.

Scratch what I said before. I turned off some stuff in the options - which mostly removes the raping of the music. The volume thing is still a bit annoying, though it's not a too big issue, since I've reached a high enough ranking to start with all the powerups. Gameplay is pretty cool, though they could have toned down the flashing lights a bit, as it gets a bit distracting.
EDIT:
As far as epileptic seizures go, that can be triggered by any number of things. I used to have epilepic seizures, followed by temporary partial paralysis, caused by sleep. That largely went unnoticed, though occasionally I woke up under my desk, unable to feel or move my entire right side. I've also heard of people with seizures triggered by specific voices, and such.