Michigan just voted to make it legal. Sounds good from the tax issue.
How's that work for border crossings? If it's legal in Michigan and legal in Ontario, do the Feds still roust you out of the car and clamp you in leg irons if you possess a bit? Would it only be illegal at the crossing point? Will they chemically sample my brownies?
As far as the federal government is concerned, marijuana is still illegal everywhere, not just "at the border". Federal agents kinda sorta look the other way in states that legalized it, but that's just Uncle Sam trying to be conciliatory without really giving away legal supremacy. The border is this weird place where not only you temporarily surrender your usual rights, and it is entirely run by federal agents. They are trained to be on high alert and all their rules apply.
Now as markbike528CBX pointed out, there is this whole interstate commerce thing going on, and things get complicated.
I live in Washington, which not only legalized marijuana a while ago. Let's think about the implications of driving around the Pacific Northwest with a bunch of joints in the trunk.
1) Driving within Washington: no problem. You'd have to somehow be stopped by federal agents. Sure there is this law that says anywhere within 150 miles from the border is "the border" and you can be stopped and searched at any time for no reason, but in practice the odds of getting in trouble are minuscule.
2) Driving from Washington to Oregon: probably no problem. Both WA and OR have instructed their law enforcement to not do anything and not report it to the feds, so the odds of getting in trouble here are just as low as in option #1.
3) Driving from Washington to Idaho: bad idea. The Idaho state trooper was NOT instructed to let these things go. They charge you with some state crime, and upon learning that you came from out of state, refer the case to the feds and tada, you get trouble from them too.
4) Driving from Washington to BC: very bad idea.