Vanguard has a calculator that helps you decide about keeping your traditional 401k or converting to the Roth. JJ is pretty much spot on. I looked at it (less than 10 yrs to retire) and it was close to a wash on converting in my case. It's recommended to pay the tax when converting from funds outside the 401k; it's a lot of dough. If you're young and Roth is available; go for it.

Life's a peach!