I have a Stoney Point Polecat Rapid Pivot Bipod System, and I never take my rifles anywhere with out it. It is the sitting/kneeling one, goes from 25"-45". It swivels, and it has rubber holding the legs together, so if you are in a rush, and have it set to high, you can spread the legs apart, or tilt it. To carry it when I walk, I set it up height wise for kneeling, then just put it over my shoulders. It is great with a lawn chair and hundreds of gophers too... :twisted:
I had a tripod, used it once, and returned it for this one. It did not swivel, so when the deer took a few steps, I had to pick it up and move it. Tripods suck on unlevel ground, and like SISIF said, you can maneuver the bipod around to make it level.
I would not buy a mono pod, my buddy bought one, it is not stable enough, unless you are leaning on something with your body as well.
I will never be without one again, and I will be buying another one of these as soon as I can.