aihtdikh

I've been playing a fair bit of Hammerfight recently, which I got with the Humble Indie Bundle 3.

It's painfully hard:
I often only get past a level by sheer fluke after re-trying over and over again.

It's extremely frustrating:
I'll pull some awesome moves, smash up a giant flame-throwing robot mano-a-mano, then get killed by a falling brick while fighting some random little minion who comes out afterwards.

And yet I'm loving it.

The swooping and smashing is visceral and satisfying.
Finally beating a level feels like I've actually accomplished something.
And, with the branching storyline, I'm already planning to re-play it to explore some of the bits I've missed.

All up, if you can get through it without smashing your mouse in frustration, it's a lot of fun. 
Highly recommended.