I have Safari, Firefox, Camino, and Opera installed on my MacBook. I used all of them, maybe not at the same time, but I use them. Each of them seems to be suited for particular tasks. For examples, when I need to visit a site that is heavily loaded with Flash ads, I use Camino for its built-in Flash ad blocker.
And when I need to pick up seats on plane for my wife and I, I open two different browsers because it seems to be the only way to do it.
Personally, I think choosing to exclusively use one browser does not make sense. I don't have to pay for any of them, so why not use more than one. Safari has some problems with some of the sites I go to, whereas FireFox/Camino will open them fine. E-mail programs on the other hand are a different story.