qzbmpc,
I asked the same question recently. If you search for Flail mower you will find mucho information. In brief I was able to gather the following information:
1) Flail mower cuts much nicer than a brush hog. Almost as nice as a finish mower.
2) Flail mower is also much safer than a brush hog as rocks and other items are not instant projectiles.
3) Flail mowers are MUCH more expensive than brush hogs and will most likely require more maintenance (blade replacement, etc.).
4) Flail mowers tend to pulverize everything they "mow" and this is usually a good thing.
Most of the people that responded to my question on flail mowers absolutely swore by them. Hope this helps.