After sifting through the 70 page thread on Flail mowers, I chose one over an RFM or a Bush Hog. I used my FIL's bush hog, and although it did a pretty good job, it was like walking around with a 12' long 2"x 4" over your shoulder. I was banging into all kinds of stuff when I'd turn. The flail's like a tiller on the back of the tractor, so much easier to maneuver.
I've read there are some significant safety advantages to a flail over a Hog as well.
I've also read that by changing the hammers on the flail you can decide what type of finish you'd like - ie "Y" type hammers for tall grass and saplings, and spoon type for a nice finish.
The PTO on mine hasn't actually turned yet, so I'm no expert. I thought this would save you scanning the "let's talk flail mowers" thread, thus leaving you more time to read War & Peace, or this years Encyclopedia Brittanica instead.....
If you're interested Agrisupply sells Caroni, which is what I bought, and it's a really solid machine.
-Jer.