I have always stayed away from battery grips because of the extra weight they entail. However, I received a Canon BG-21 grip with my new Canon 6D Mark-2 and I love it! I am a rather large man and the camera + battery grip really fits my hands quite well. It "seems" that I am able to a note steady grip with the grip than with just the 6Dii alone.
I have not tested any camera/lens combination with and without the grip but it just seems like my hold is more steady.
This is especially true when I am using a long lens like the 70-200mm f/4L IS or the 100-400mm f/4.5-5.6L IS ii. However, the EF 35mm f/2 IS pairs very well with the 6Dii and battery grip. The balance is perfect and that combination is a joy to shoot...
I can foresee that the above combination will develop into one of my favorite camera/lens combinations.