As for the M570, I have one. it's not horrible, but I have little use for scroll wheels and (being a Linux user) want my third button back. Also, the top curve of the shell is different from and in my opinion inferior to the T-CH11 - it's like the designed it for someone with a child-sized right hand.
I am Linux user too and find the scroll wheel scrolls just fine. It also works as the third button, i.e. you click the wheel. It does take a little practice at first not to turn the wheel as you press is downwards but didn't take long to get used to that.
The RF circuitry is weak; signal can be blocked by the mass of an adult human.
Mine seemed OK - I had the M570 on the top of the desk and had the receiver under the desk where I have a KVM switch. Interestingly the receiver worked fine in the KVM switch despite Logitech not supporting that mode of operation but I did have to put the receiver into the hub port of the KVM, i.e. the one that doesn't have the KVM looking for hotkeys but is just a plain USB hub switched to whichever PC.
I've heard M570s tend to die quickly, but not had mine long enough to refute the claim.
Mine has certainly died, in the sense that if I turn if off and turn it on again it will work for a few second and then stop working. New batteries does not fix the issue. I have probably had it a couple of years. I am now using a Perrix one which is almost identical.
Thumb ball users, how's cursor stability on a thumb ball during energetic spam-clicking while simultaneously moving the ball (or at least not lifting your finger off of it)?
I don't find it to be an issue as there is no moving of fingers. On a typical thumb ball there is only one hand position with the thumb on the ball and the fingers on their respective button so if you can move a finger without moving your thumb it will work fine. That certainly my experience.
I did use the Kensington Orbit before they introduced the version with the scroll ring and didn't really get on with it and I wondering now if the issue was the need to move fingers around. I will see if I can find it again and give it another go.
My first trackball was the Logitech Trackman Marble FX which a number of other people have mentioned. This is a finger ball but unlike the Kensington ambidextrous ones it is shaped for the right hand. The way I used it, which is what I think was intended, was my thumb on the left button, my ring finger on the right button and my pointer and middle fingers on the ball. The centre button was less convenient but not hard to get to as that involved moving the thumb (it was on a vertical face so certainly not intended to use the pointer finger on it. I dug it out to describe here and was reminded why I stopped using it - the left button has become dodgy.
Before it developed the fault I liked using it. Whether because of the bigger ball or using the fingers instead of thumbs it seemed easier to have a decent rate of movement and still be able to position accurately. It is much less like using a mouse, though than the thumb balls so you have to be a little patient at first.