I'm a fan of some of the original but outdated thumb trackballs by Logitech, so I thought I'd give their new one a try.
The appeal of the new one is better resolution, extra buttons and a wireless connection:
The wireless connection works perfectly and causes no delays. I took the trackball to a friend once, and there I had to plug it in one of the USB ports in front of the computer, because the back was too far away or there was too much interference. Other than that I have noted no difficulties. Some people will dislike the fact it isn't bluetooth, but USB. I don't really care. The tiny USB receiver does the job, and can be used to unify Logitech devices I think (I don't need that).
At first the ball action didn't seem smooth, but that quickly went away after half an hour of usage. It's very smooth. It's also very easy to clean inside, which I have to do once or twice a week to keep the action smooth. You poke your pink in the hole at the back to pop out the ball.
As for a thumb trackball, the mouse movement is good and the ergonomic thumb ball action is very good, better than a mouse or other trackballs because there is no hand movement. However, I have tried many pointing devices and in general I think any modern ordinary mouse is still better as in faster and more precise than any trackball, and this trackball is no exception. It's difficult to find the right speed/acceleration while remaining precise and not overshoot your target like a small icon. I purely like it for ergonomic reasons.
If you use one of Logitech's older thumb trackballs, you will welcome the extra buttons, which I use for back/forward in the browser. I like those. All buttons though are designed in such a way, with tiny gaps and such, that crud will accumulate, you can already see some of that in the pictures.
The main failure of this trackball is its scroll wheel (which also is the middle button), which fails in two ways. First, unlike the modern Logitech mice, it's a one speed wheel with little bumps when you scroll it. Nice for some stuff, but for other stuff you like to switch to the smooth scroll wheel mode as sported by the mice. I'm not sure why they put an outdated scroll wheel in there.
Second, compare the placement of the scroll wheel on the M570 to the first picture. There is no dedicated finger for it! You have to move your index finger to the right from its resting position, or your middle finger to the left. I do more scroll action than right mouse button action, and this is a big fail for me because my fingers are not in an ergonomic position when I scroll. My ring finger is not used. I could move all my fingers one button position to the left, but then it gets crowded on the top of the mouse. Ergo design fail. I still like the trackball, but it is not as good as it could have been.Pros:
Wireless (for what it is worth), good reception on small distances, tiny receiver
Very good battery life
Smooth trackball action, very good for thumb trackball
Easy to clean the ball and pearls
Extra buttons compared to older modelsCons:
Gets dirty around the buttons
Simple, outdated scroll wheel
Scroll wheel placement between two fingers, no dedicated finger
(mainly because of the scroll wheel issues)