The New York Times did a blind taste test of vodkas without actually tampering with them. They picked a bunch of the big name brands - Ketel 1, Belvedere, Grey Goose, etc - and at the last minute, threw in a bottle of Smirnoff just to have a bottom.

Smirnoff won.

Basically, their conclusion was that for alcohols that are supposed to taste like something, like gin or tequila or Scotch, it's worth considering getting "better" quality. But vodka's supposed to be practically tasteless as it is, and you're usually drinking it mixed with other stuff. Filter the rotgut so you don't make yourself ill, or just get Smirnoff.