I got to Satan on my first try, and midway through his first form, the music broke and started playing really slow and loud, like some kind of horrible grinding synth. Then, the framerate dropped to probably around 5 FPS. It was literally unplayable, as it took a few minutes just to move from one side of the screen to the other. After the headache of fighting through such horrible play conditions, I got to the final form and would have killed him, but when he had about 1/4 of his health left, the game just instantly quit out.

I can't imagine I'm the only one who had this issue, and I really hope it gets fixed, because otherwise I won't be playing anymore of this game, knowing that that will likely occur again.

I got the horrible grinding music but the last boss said it was The Duke of Flies and it turned out to be The Headless Horseman.
 
I thought it was supposed to be Isaac going insane or something because that is what the ending implied. I guess my game just straight up fuckin' broke.

I had also ran into massive slow down and messed up loud music during the Satan fight. I think I killed him, life bar was empty and foot was still on screen, but so was the statue of Satan,and then the game crashed. I hope the next update isn't too far off, there seems to be a lot of things to fix from the Halloween update. Beyond that there's still issues with the original portion of the game which is sad because the game is awesome otherwise.

According to Edmund's twitter, there will be a patch tomorrow to fix some of the issues.

http://twitter.com/#!/EdmundMcMillen/status/131257653266620416

I just fought the headless horseman instead of Satan at the end of Sheol. During the splash screen that displays the boss name it said I'd be fighting the Duke of Flies, though it was the headless horseman. Not sure if that's working as intended, but I completed Sheol with Isaac and it unlocked Mom's Knife.

The update to fix most of the new bugs seems to be up now, It's downloading as I type this.

Apparently getting the charge beam (forget what item it is) with chocolate milk (which allows charged tears) causes some kind of massive glitch where you can take out almost any enemy with invisible screen filling beams. But when I got to Satan I had that FPS/music glitch and I discovered that my super beams couldn't hurt him.

Just so people know, I just beat Satan with brimstone, so it seems like the issues are actually fixed now :D