(By throwing out, I mean tossing in the closet in the hope that one day I'll get around to soldering on new switches.)
Out of the box it was way, way too sensitive. I had to turn down the DPI, which took a few hours of finagling with the firmware. Some of the documentation was a little sparse, and I chased down a few rabbit holes which got frustrating.
Now that it's configured it's mostly usable, but I still struggle with precision compared to the M570. I find myself a few times a week clicking the wrong icon on a bar, or the wrong item in a list. Or my hand tenses up because I'm working extra hard to hone in on the right place on the screen. These are problems that I never had with the old Logitech, and in fact, I still have a working one on the desk that I jump to occasionally when I need real precision.
The M570 has 3 little nylon-looking bearings that the trackball sits on. The Ploopy instead has 3 large wheel shaped rotators. Every once in awhile the trackball will seemingly catch a hitch on the rotators when I'm trying to make precise movements and I'm unable to get where I need it to go unless I make coarse adjustments to move the rotators off the hitch, then back a different path to where I was before, finally to make the precision movements I needed. Not the end of the world, but another minor annoyance. This has happened less lately so it's possible there was a "breaking in period" with the rotators.
Overall it's a decent device and after a few months I am mostly happy with it. Durability is TBD.