I use Gentoo. I get a lot of smack about how long it takes to set up the thing initially. "How many weeks did your install take?" is a typical example. But once it's up and running, assuming you keep it updated, it's pretty sweet.
Linux overall is great if you find yourself doing a lot of code development -- which is a tendency in math, physics, computer science, etc. And it's marketable: many physics and math internship programs (don't know about other fields) are now listing UNIX as required/preferred experience.