Having now taken this flight several times a year since its inception it's become harder to upgrade every time I've flown (As Premier Executive I was about 30th on the u/g list and 80% of an A320 boarded in Group 2 or higher last time I did it), and the timetable shows it going to two a day on some days in summer 2013, with the addition of a morning flight.
So at least from my anecdotal impression this flight shows that sometimes you gotta stick in a market for a while. MSP seems a secondary market for UA that it's worth taking seriously. Going inter-continental DL offers you non-stop to Tokyo, Amsterdam and Paris (with good connections beyond), and non-stop with no connections to London. UA with its flights to SFO, IAD, ORD and EWR offers a domestic connection, and then the international flight to the secondary European/Asian cities. Not clear if you're doing a lot of international flying that DL is really much better.