I have 5 batteries, and only 2 are from Oly, the others are off brand, but good and reliable. I don't see a difference in performance, or in charging status accuracy by the cameras.
I like the extra battery grip very much, it helps immensely when shooting verticals, especially with the 25mm lens from Panasonic, or the Oly 75 f1.8. Even when shooting with my newly acquired Oly 12mm f2 lens (on tripod) I tend to use the grip. I am not very careful with my gear (I don't tend to baby it) and since I shoot a lot on the coast with rocks and sand, and I tend to put the camera on the ground often, the grip provides a bit of extra protection. The whole thing just feels more robust with the grip.