Watch out for silicon lube on the floor on the shower when you wash up afterwards . . . can be quite slick with water on it.
But other than that - silicon is LIGHT YEARS beyond water based lubes. Water base lube is guaranteed to leave your ass feeling sand-papered. Silicon stays slick, slick, slick.
Frankly, if one of my straight friends told me he'd decided to go gay, my first advice would be: silicon! (I.D. Millenium is awesome, BTW.)