Just bought a Wessex 120 Flail that I tow behind a Honda Fourtrax.
We have 1 1/2 acres, mainly bracken and this year Himalayan Balsam (Evil plant). The first time I tried it the flails were set quite high as the bracken in places is as tall as me, 6ft. Because the Quad is narrower than the flail it just tends to flatten it however, a broken fern is as good as cut.
I will lower the flail when i've worked out where the rocks and gorse bushes are but so far it works well. My only nigle so far is with the 'kill switch', because I have tall bracken the first day I used it something caught the curly wire and dragged it under the flail, mashing the wire and damaging the switch (Wessex please note).
I solved this by attaching a 3ft pole to the flagpole mount on the quad, re-wired the switch and attached to the top of the pole. Problem solved. All in all a good peice of kit that will certainly get some use on our patch of shang ri la.