I'll start by saying I am new to any off road riding of any kind. Bought a Giant Contend AR 3 a few months back and got a second set of rims to setup for off-road rides. At first, I bought a set of Schwalbe Marathon Plus 38c (actually 40-622) because I am planning a trip to the GAP Trail and have heard the Marathon's are great for all different kinds of surface touring.
I rode them once at a lake that has everything from gravel to dirt, grass, sand...you name it. Though they did OK, I didn't feel as confident as I would have liked.
Enter the Ultrabites. I got them and set them up to about 55 psi and headed back to the lake trails. AWESOME. While the first ride made me wonder if I should get a mountain bike (not many hills here) these tires were absolutely perfect for all of the different surfaces. I am thoroughly happy with them and found myself willing to ride 4 to 5 miles faster in some of the sections.
I highly recommend them.
This tire flat out shreds in the dirt. I run 35 PSI on the 700 X 45mm tires. It's stable in all conditions and allows you to push your gravel rig in single track with steep descents as well as give you the traction to climb those steep ascents.