Just a few experiences I know of the situation of crossing with a DUI or for that matter even a possession charge.
I have a friend who was going to be outside of Toronto doing research. He had this exact concern regarding a DUI that was downgraded to a lesser misdemeanor, non DUI classification (yes, I know a DUI is a misdemeanor, but has a different social stigma). When going over the border, the Agents were aware of his ORGINAL charge, as well as the change in the charge... I found that interesting as running a criminal background check (he had to for some governement research project requiring classified status). He was granted his VISA, and was able to go back and forth a few time with no real issues. Canada has the right to refuse any traveller with a criminal background, same as the USA.
I have another friend that had a fucked up ticket (wrong info on ticket, never received a summons due to the incorrect address on the ticket), who was taken in on a bench warrant. Luckily everything was dismissed, and he was told its no longer on the record. Now he had a girlfriend in Canada, and once he was asked by an agent had he ever been arrested. Knowing that the ticket was dismissed, and no charges were placed on his record, he said "No". The agent proceeded to call him a liar and asked him about the incindent that was in the system. Obviously this dataset has a shit ton of information on individuals who have been arrested at one point or another. He was told from there on out to respond affirmative to the quesiton, and to further explain if need be, this was after being chewed out for a bit by the agent.
Now I had a fucked up personal experience myself. I was under 21, going to the good ol' Canada to drink with some friends. I had an empty seat and was leaving another party when a guy I didn't know well wanted to come. Well long story short, he was a felon, on parole, had no drivers license (even though he drove a BMW), which was discovered when the agents were handed his ID card and not a drivers license when he was furthere questioned. TWO HOURS in detention station while he was being questioned and my car ripped. We were denied access to Canada. I wanted so bad to drive him to Brightmoor, leave him there at 1:30 am, and let him find his own way back to Ann Arbor.
Just my two cents and expereinces.