Open Country A/Ts were one of the tires I've run before. The M55s are better in every way except noise when the windows are down, and speed rating (M55s are only rated for 89 MPH, but they hold up just fine at much higher speeds for short bursts like when passing). I also get double the life out of the M55s compared to the Open Countrys.
As Marty mentioned, the open lugs are great for the heavy sloppy stuff. But also important is they don't get hard in low temps like the Open Country tires do. The M55s stay nice and pliable even sub-zero. Also they have siffer sidewalls so they are nice and stable with a trailer even with their open-lug tread.