Some times I'll get "Any alcohol or tobacco? Any firearms, pepper spray, or mace? Have a nice day." Other days it's "What was the nature of your trip? Have a nice day." Other times they'll want to know what I had for breakfast last Tuesday and every penny I spent. Overall, male guards give you an easier time or are more joking/friendly. Female guards seem to have a chip on their shoulder, though not always.
I've gone from being an 18 year old kid visiting his Canadian girlfriend, to being on a student visa, to being a permanent resident, married to a Canadian, and now I have my citizenship application in. I've gone both ways with any combination of US and Canadian citizens in the car, and with Ohio and Ontario plates. If you're young and alone, you're going to get a lot harder time than if you're a married couple. If you're just dating someone or with a group of friends, the questions will be more prying too with the "how do you know each other?" and "how did you meet?"
I've never had a criminal record, so I've never had that complication at least. From what I know though, prepare to be profiled. If you contact the border ahead of time about that factor and have your Ts crossed and Is dotted on everything else, they won't throw the book at you. Even if it's legal to do so, not bringing any beer, tobacco, or firearms with you, and leaving the radar detector at home if you're going into Ontario, is going to give you an easier time, as the only point of contention will be the DUI. If you don't give them reason to give you a hard time, they won't give you a hard time. If you're unprepared or ignorant and walk in as a deer in headlights, they're going to ask you everything they can, and probably send you inside to straighten things out.