I have grips for both my 5d and 6d, but I only use them as needed. The 5d is a genuine grip and I normally take if off if I need to carry my body for a while (walking around the city etc). I do use it though if I decide to use a heavy lens when doing portrait sessions etc. In essence I use mine for balance and not extra battery life - but I could see if you were at a sports day, car racing etc the extra battery life would come in handy. it is less than a five minute job to put it on or off. I often carry it with me in the car just in case I decide I want to use it when I am out and about.

I have a non genuine one I bought for the 6D but rarely use it. The main reason for me is the battery tray little lug on the end has lost it's friction is falls down (which annoys me). The 6D is a light camera and mostly used by my wife, so the extra weight of both the grip and the battery just gets in the way for her. Bare in mind she only uses a 24-105 on this camera.

I also have a 1Dx which has a built in battery grip - I just know this is a heavy camera before I pick it up - so it is not a body I take with me when I am acting as a tourist. This body is the perfect shaped and weighted if you are using large heavy lenses.