My hometown of Spokane, WA just opened the first legal marijuana store all of two days ago. They sold out right away, so... the demand is definitely there.

What will be really interesting to see is how the state can compete with the existing growers/dealers RE: price. Right now to buy marijuana legally in Spokane it's about $700/oz. Your friendly neighborhood dealer will have it for $200. As it is now, there's not a lot of incentive for the old growers/dealers to go legal, because it'll cut into their profits (and their customers won't want to pay more anyway).