Halsey, here's my opinion from using a small flail shredder/mower about five years. They're expensive, but less than buying both a rotary and finish mower. Time is also expensive and mine saves time. Mine uses hammer knives, are they're expensive too. It takes time to change or sharpen them, but I don't need to do it often. The mower is heavy, but my 4100 handles it easily. It's too heavy to make sharp turns while mowing, but lifting it slightly solves it for me. So far this also describes my tractor ... expensive, heavy, makes work go quickly, and requires maintenance. And so far I still love my tractor and my flail. /forums/images/graemlins/smile.gif
I wanted a nicer cut on my field than a rotary would give, and the flail cuts super low when I lower it. I mow a field and orchard, it mows & mulches just about anything in its path, include tree prunings and, a month ago, an old plastic tray hidden in the weeds. They can throw rocks ... forward, and right on the ground. Each hammer knife weighs about 2 lbs and when they hit a rock, the rocks break and/or fly. They can't fly up and hit me, and I don't mow with people in front, so it's pretty safe.
Mine is stored on a rolling platform, so it's easy to attach even though it weighs plenty. I don't disagree with anything others have said, but for me I'm happy with it. Best of luck.