This has to be one of the most amazing Anime series ever made. It combines some of the funniest and absurd physical and intellectual comedy with a powerful drama and sense of what I would consider to be Horror in the emotional sense.
Vash has to be one of the most contradictory characters in any anime. A Gunfighter who hates killing. Nearly Immortal, yet vulnerable. Power to destroy all life on a planet, but upset at the death of a bug... It really is amazing.
If I had to choose any series which has impacted my own sense of ethics, this would be it.
The concepts of Life and Death, Good and evil are brought to the line in the grey... Not the blurry grey area, but the pencil thin line within the miles of grey.
The goodness of Milly, the callousness of Nick, the pride of Meryl, the hate of knives, and the love of Vash... Though through it all, even the smallest character is "Fleshed out" with a multi-faceted personality. It really is amazing.
Though I would not recommend this for young children, I believe that rational adults people can only come out better people after seeing it.