I've gone the first week in March for the last two years and love it. SUCH low crowds. Wait times are a breeze and we've been able to get everything done that we wanted to in two days.
It definitely gets chilly at night so bring lots of layers. The afternoons aren't bad at all though. Maybe a long-sleeve top or a light sweatshirt necessary. We've been lucky enough not to have much rain at all. We didn't experience any in 2009 and only got a little sprinkling this last March.