So, I have reviewed some ... different types of accessories in the past. And I'm usually surprised by what they can do against what I thought they were for.
I was asked if I wanted to look at this new ergonomic 'mouse' and I thought, why not. I thought it might be good for my Pixelbook. Have a look see (and don't mind the messy computer desk)
So, I was disappointed by the fact that it didn't work on my Pixelbook, but it worked instantaneously on my Pixel 2. Weird. Not that I'd probably use it for a pointer on my phone. But it gives me the idea now that I should try it on my Android TV box (though, I'd be limited by the cord length).
Anyhow, let's talk about this thing. First off, I was amazed that it was $225CDN (I found it on Amazon.com for $265USD). Pretty pricy for a fancy trackball.
The design was still curious to me, so I pressed on.
The idea here is that you can attach it to the 'bottom' of your keyboard so that it can act as a wrist rest and then it is in the position to be a sort of trackpad. Your thumbs are right where they'd need to be to interact. No, having to take your hands off the keypad and move it over to the mouse back and forth.
The main purpose of this is to replace your mouse.
There's a roller in the middle that acts as your scroll wheel, a left and right mouse button (the 2 little ones above are 'back' 'forward' buttons for browsing. 
The big feature is the cylindrical tube at the top.
Essentially, it reminds me of the roller thing on the old typewriters, you can roll it up and down and it moves your mouse pointer up or down. Combine it with the fact the tube sits in a trough and can move left and right, you now have horizontal motion by moving it. Pushing down on the tube will act as a left click too.
Pretty neat. It feels innovative when really it's probably more 'nostalgic' in a sense...
But how does it work?
Well, it takes having the right keyboard for the height to be appropriate to start. The moving of the mouse pointer with the slider works exceptionally well and more intuitive than you'd think actually.
The biggest downside is that we've just been trained so much to use the mouse that even though the slider is RIGHT THERE under my thumb I still would reach over to the mouse. It takes some training to get into and even with it for a week I kept doing it.
My co-worker said she has a friend who has one and they really enjoy it and it's helped with their RSI (repetitive strain injury).
For me, the other issues were that if I tried to press down to left click it would slightly roll the tube and move the cursor, so again, it takes some real training to get to use.
Your use case would need to be pretty specific, especially at the $200+ cost.
What was surprising to me was that it took so long for it to recognize and work on my work's laptop, yet it worked instantly on my Pixel 2. It never recognized on the Pixelbook. Ah well.
In the end, my review is that it's novel, but not so necessary.
Source: Ergoslider