It didn't take very long (about a month) after I started using my new E-M10 Mark II before I realized the ECG-3 was going to be a must-have accessory. I am a man with average-sized hands, and I simply could not grip the camera securely without this accessory. I also have carpal tunnel syndrome, and "pinching" the camera with two fingers and a thumb wasn't going to work for me. If you have smaller hands you probably won't feel the same way. Now as to the fit, feel and function with the accessory grip attached:
Essentially now it FEELS as though you are holding a full-size DSLR, but the camera is still smaller, even with the grip. It does noticeably change the height of the camera (about 1/2"), and the depth is slightly enlarged without a lens attached. However if (like me), you tend to leave a lens on it (any lens, really), the depth will not change in terms of fitting it into a bag or pouch. The height will. The fit and finish of the ECG-3 is excellent. The two-piece design fits very tightly and securely, yet allows quick and easy (if somewhat awkward and definitely two-handed) access to the battery/SD compartment. How often are you actually taking out the battery? Unless you are shooting all day, probably not at all until you get home. When you do, you can simply (with a sliding lever), disengage the removable part and you are there. Aesthetically it integrates with the camera seamlessly, to the point of being almost invisible to the eye (once you are used to the taller profile of the camera with it on). Very nice there. So functionally, this is pretty much a 5-star product in my opinion. It's too damned expensive by about $20, though. There are at least two other aftermarket grips available for the E-M10 Mk II, and I have tried neither, but based on reviews from those who had, I opted to hold my nose and pay the price for the Oly grip. I have not been disappointed. Although I don't do much tripod shooting, there is nothing in the grip to get in the way of that, if you do. One small caveat: With the removable part disengaged and removed and the "fixed" part still attached to the bottom, the camera will not sit flat on a surface, so be careful if placing it down near the edge of a table or counter (this is a non-issue with the removable part attached to the camera -- it sits just as flat as it does without the grip).
Bottom line: If you are buying an E-M10 Mk II, by all means try the camera first without the accessory grip. However if your hands are relatively large, or you are used to shooting with a larger camera, you can pretty well bet you are going to be happier overall with this grip. I am.