Very creative gimmick that's done well. I really like the addition of the jump button, it helps out a ton and it introduces another way to dodge the flail and the bullets. I also like that you have to use the flail to hit the switches and break the walls, so positioning is important. The graphics and presentation looks great too.
The game could definitely use some checkpoint in some rooms, particularly the big room before the enemies are introduced. I also don't think the flail should pull you down into the abyss - since you don't have full control of the flail it's pretty easy for it to go just a little too far then fall down. It's how I died the most. Plus, trying to dodge the flail and the enemy bullets is already quite challenging enough.
Overall, great game and great mechanic. I managed to get to the boss fight but couldn't beat it, but I had fun with the game.