I use a 6' Frontier (Deere) sickle bar. I'ts new, have used it twice. I use it on the dams, around the ponds edges and to beat back the encroaching hay fields after they have been mowed.
I backed off the 7' one to a 6' one for cost reasons. What I forgot to take into account was that on the dam I need to be 1-2 feet back from the edge. When mowing around the pond edge, I need to be 3' back so there is no chance of the bank collapsing

. So in retrospect, the 7' unit would have been nicer. That said, it's very nice to rotary cut the dams, and then beat back the stuff that would otherwise grow over and lean into that cut path. For the hay fields, I keep the branches back so that cab tractors work. The sickle bar lets me cut under those branches without banging up the cab.
The one I have is maintenance intensive, it needs to be greased every 2 hours of use. But two hours is more than enough to do all the cutting around the ponds, and it takes about 2 hours to go around all the fields. I suspect this will be a 4 times a year on the ponds, and once a year going around the fields event, so that works out OK. It's odd to have an expensive implement that might only get 10-12 hours a year on it, but the work it does could be done no other way.
Last time I used a sickle bar was on an 8 HP Wheel Horse tractor when I was 12. It was a 3' bar and it jammed all the time. This new sickle cut through a stand of gum trees saplings 1 to 1.5" in diameter and didn't even flinch. Guess 40 some years of technology, 7 times the horse power, and the heavy duty bar teeth make the difference

.
Hope this helps, sorry I don't have more than a few cuttings of experience with it to share with you.
Pete