In the past 2 years, I have cleared about 5 acres of wild roses on a hill with my Yanmar YMG1800d (18hp, 4wd) and a Howse 4' rotory cutter. Most of these were between 8' and 15' tall and as wide as 30'-40'. Like Gary said, I run with the FEL low to help clear the blinds a little and give me warning of what's ahead. The worst part for me is getting stuck with the thorns on the side that I haven't cut yet! But the feeling of knocking all of that bad stuff down and seeing it as a nice grass-covered field the next year is great. This was all done on a slope, so I tried to attack it from the uphill side and mow going down. The times I had to cut sideways were the butt-puckerest, and I always went in low-low to give me some sense of control.
-Frank