We have the biggest size of the walk behind model. 30 inch blades, I think, and I believe 18hp Kawasaki engine.
We are extremely happy and satisfied DR owners. It is a very well manufactured piece of equipment. You can tell that it's been around long enough as a product that they've added little tweaks to the design to keep common breakages and wear and stress from becoming a problem. No plastic or crap parts. All very solid. Easy to clean, easy to maintain.
We started out with the default multipurpose blade that mulches, and it does great on bushes and sapplings up to 2" or so thick. The heavy duty blade is much better for things up to 3" or so, and it does not mulch if you have kudzu to mow. The multipurpose/mulching blade, though, just makes stuff disappear. It's amazing.
The DR is definitely a large piece of equipment that requires real human power to operate. For example, it does not turn on it's own so you have to use your own power to lean down on the handles and turn it using the torque of your body. You need similar torquing resistance when you get into the seriously thick stuff.
The mowing is at 4" off the ground, so many small rocks and stumps are easily avoided. If your land is like ours, though, you have to scout the area for larger stumps and rocks so you don't tear up your blade.
You might want to make sure you have access to an impact wrench for removing/changing the blade.
I had never heard of the Simplicity that someone else mentioned, so I did not do a comparison there.
Hope that helps!
Dan