So I've been using the MX Ergo for about a week now and I can say I like it! This is my first time switching over from a traditional mouse to a trackball. It was a lot faster to switch than I thought, I got used to it quickly. I also find it a lot more comfortable than a traditional mouse, especially thanks to the 20 degree tilt, it's very handy.
One problem I have is a lack of smoothness, whenever I try to move horizontally with the mouse there's a sense that I can't be as precise as I would like to be. The ball kind of "hitches" and doesn't move smoothly. Apparently this only happened in some batches of MX Ergo, so you might not have the problem. I am going to call them about it tomorrow and see if I can get a replacement mouse. Overall though, if I can fix the hitching problem, I see myself using this thing till someone makes something better!