I was up there a few weeks ago, POTHOLES all over the place. I do have a suggestion, when you get up to the border, get off at the exit to Rouses Point, get gas (station to the right) and then continue east until you go about 2 miles (I think 276), make a left and cross the border there. After you cross the border continue going until you see a flashing yellow light, make a left there and go west until you pick up 15 north. Come back the same way. This can sometimes save hours on the border specially during the high season.
As has been mentioned, the roads in the city are really bad although taking 15 north isn't all that bad excpet for the construction they do in the summer. Also, often there is a lot of wind (from the west) until you get up the the bridge into the city and it is a really messed up intersection. After you get over the bridge, it is ever more screwed up. They have signs but if your heading north you need to get off something like exit 57 and then get in the left lane. I will say, it will be worth the trip