Without doubt, when it comes to comedies, "Kiler" is one of the best things that modern Polish cinema has to offer. The story of cab driver mistaken for a professional assassin is really hilarious, from his short stay in prison to the surprising ending - we get tons and tons of laughter. Brilliant characters' creation really contributes to that: you will fall about when fat gangster Siara with his redneck mentality or wannabe-Dirty Harry policeman Ryba appear on screen.
The plot itself is very well-written, containing some unexpected twists; enemies deal with each other rather in entertaining game of wits then with shooting or brute force - if I remember correctly, the single actual death in the movie is a result of funny accident with salt container. You wouldn't expect that from a movie about an assassin, would you? ;) Music is great - it is performed by Polish well-known "poetic rock band", Elektryczne Gitary (The Electric Guitars). Their lyrics comment the action of movie in an ironic, sarcastic way; unfortunately, they would be extremely difficult to understand if you're not a native Pole.
All in all, that movie is a must. Correct me if I'm wrong, but no better Polish comedy was ever produced after "Kiler".