The section of the NT between Bismarck and Fargo, ND is new, so I cannot commnetn. I enjoyed the NT route through MN. Took the "long way" up to Grand Rapid. (Ended up in Davenport, IA instead of Muscatine and crossed into IL there.) While I liked ND a lot and have since been back there to ride, it was nice to see some trees again in MN.
Lake Itasca was really nice. You can walk across the Mississippi River there. Camped at a couple of other places on lakes. Saw a bear cub in Malmo and then camped at Mile Lacs in Isle. Also camped along the river in Palisade. Grand Rapids is the home town of Judy Garland. Went off route a bit and spent a few days in Minneapolis. In IA, the route passes through Dyersville, where the baseball field in "Field of Dreams" is.
Probably not the best route if mosquitoes really bother you. They were a constant issue in most places, including the hostal at Lake Itasca. It was also God awful humid many days, but I suppose the alternate route has some high humidty sections. On the other hand, there were actually a few days when it was on the cool side. We hit Fargo on July 6th, so that would have been early to mid-July.