In practice, I checked my website's usage statistics, and see less than 1% are IE6 and below.
I ask because I recently updated the site, and according to browser simulations, it appears the navigation tabs at the top don't show up on IE6.
I can't figure out why. And if I make it so it works on IE6, something else breaks. (And I should probably be worrying about how it looks on iPhone and friends, shouldn't I?) Not that I've got hordes of people beating down the doors to visit the site, but still.
Which web browser(s) do you use?