So you guys are former rotary cutter owners that switched to flails, huh? 
 
I'm getting ready to buy a rear mower. Still time to change my mind. 
 
What's the advantages? Tougher, less debris thrown around? more reliable? 
 
They do seem like a more professional tool for the job.
		
		
	 
Hi,
Never owned a rotary, but have used them a fair bit.  
Advantages of a flail are:
Tougher - Maybe.  Rotary's can be pretty tough too, but the flail's seem BOMBPROOF!!!  Both are very simple by design, so there's little to go wrong, and they're easy to maintain.
Less Debris - For sure.   I'll summarize parts of  the 60+ page thread on flails.  Rotary's can, and have, killed people with flying debris.  From what I've read, a flail will not do that.
Shorter rear length - The flail is like a tiller on the back of the machine, whereas a rotary is like carrying a 10' 2x4 over your shoulder and taking out your buddy with it!!!
Cut quality - I've been happy with the little I've done so far, and from what I've read that a flail, with the right knives/hammers, can leave as nice/almost as nice, a cut as a finishing mower.
Brush Cutting Ability - There are several examples on TBN of guys clearing some serious brush with a flail, up to 2" saplings I think.
Flail's are expensive, except for the Caroni!!!  A Frontier or Landpride can be well over 4k, but the Caroni is only around 1600$.  Call Agrisupply and see what their direct price is (phone price is cheaper than internet).
I wouldn't hesitate to go with the flail.  If you want to mow golf course greens, a 3pt rear finisher isn't what you want anyway, and a bush hog is DEFINITELY the wrong choice.  For that you need a zero turn type unit anyway. 
Hope that helps.
-Jer.