Hi Fabio - Michael was clearly trying to kill me with the 'gifting' of his old sickle-bar attachment. I bolted it on and enthusiastically went out to work. An hour later, my hands and wrists were numb from the vibrations, and I realized these things are best given away to unsuspecting friends.
I have since discovered the 'double-acting' sickle bar mower from Earth Tools, which costs 2x as much as the single acting one but has virtually no vibration & is super awesome, I use it all the time. The single acting ones are truly terrible, please, no one waste their money on them.
PROS: It cuts a 4' swath (much faster than a 2' rotary mower), you can get a 6' also but wouldn't work for me since I'm also mowing between closely-planted berries..but if you had big areas to cover, imagine how fast you could mow with a 6' pass. The BCS unit (or Grillo which I bought b/c $1k cheaper) can offset it's handlebars to one side so the operator can be somewhat out of the way of branches, while the mower sneaks underneath a bit. & with the reach of a 4' bar you can get under the trees very effectively.
CONS: Unfortunately it is fairly easy to hit trees with this if you're not really careful...so be extremely careful using this setup on young plantings. I actually cut a 1" apple tree down, it cut it clean through. (ouch..) But for older, bigger trees, not really an issue..they can take an occasional hit.
This is a hay mower, lays down the grass clean-cut at the base..does not chop it up at all. This may be a pro or con, depending on your situation.