I generally find stick fights overly repetitive.
But I was pleasently suprised by this one.
Though I was slightly confused by your introduction, things really picked up when he fell down the sewer. Frankly the introduction was not essential, but it was animated well and it was nice enough looking.
The music: It was suited for this type of flash and did its job of creating a tense atmosphere well enough.
Animation: Really fluid. A lot of people make jaggy stick fights that aren't smooth enough to capture that "awesome" factor. obviously you did capture that factor, and the fight was nice and badass.
The ending was fine, either way we knew someone was going to die (like I'd spoil it in a review).
I associated personalities with each character through your expressive animation, and I got abit attached to them throughout the flash.
A bit of added length to the flash might make it better.
Keep up the good work.