I rented an Orec Cyclone Flail mower and wow, it's kickass. Used it for my backyard, about 1/2 acre.
BEFORE: AFTER:(slightly diff angle but you can see the two same small trees on the left side)
THE MOWER:It's amazing. Pulverized everything that was about 2-3" or smaller. Anything bigger than that, I used a chainsaw to fell it down and pulverized all of the limbs.