I also know several US citizens that have been denied entry into Canada because of a DUI in the United States. AFAIK It's not a hard fast rule, and you won't find out until you get there.
As a Canadian, I was surprised someone would be denied entry for a DUI. Since I looked this up I'll summarize:
Essentially Canada can prevent people who have committed what would be indictable crimes under Canadian law from entering. DUI is particularly strange because it can be either an indictable (serious) or summary (less serious) offence, and in Canada this depends on the circumstances and the way it's prosecuted. Coupled with the varying laws in different states regarding DUI/DWI/OUI/etc, and presumably to prevent the border guard having to act as a Canadian criminal court (to judge how it would be handled here) they just always treat it as if it was an indictable offence.
So even if you're not driving and not intending to drive, the fact it's a potentially indictable criminal offence is what prevents entry -- nothing at all to do with it being a DUI.
There appears to be a couple possible ways to be admitted despite a criminal record[1], by getting either a Temporary Resident Permit (TRP) or being granted Criminal Rehabilitation. I'm not sure what's involved with either, or how likely it is to be approved, but they apparently issue around 10,000 TRPs a year.
Yes this has been a problem for several people I know. There is an arduous apology form process one can go through but most people just give up because it is so embarrassing. I've had more complications at the US-Canada border crossing than at any other crossing I've ever taken.
It's particularly interesting given that having a DUI on your record is not disqualifying for entry into the USA. The US denies entry for what it deems crimes of moral turpitude, a relatively nebulous concept, though one a DUI does not fall under.
Canada doesn't exactly have an open border.