As much as I love the compactness and light weight of the stock OM-D body, I think that the grip and battery pack is marvelous. When you mount one end of the strap on the battery pack strap mount, the camera is in portrait orientation around your neck and suddenly the position of the thumb grip and power switch makes sense.
The naked camera didn't seem to sit in my hand properly because I am used to resting my thumb near the middle of the camera, but with the grip you can shift your hand a bit and the thumb falls naturally on the thumb rest. The seemingly stupid position of the power switch, which requires dislocating your thumb when in landscape orientation, now makes eons of sense with the camera in portrait. With it around you neck you can flick the power on at the 'top' of the camera. And when you hold the camera by the battery grip, your thumb is right next to the power switch.
I think that the OM-D's ergonomics were designed with the grip as an important part of the camera. The location of the power switch simply does not make sense otherwise. And with the camera hanging in portrait position it is still easy to grab the grip from the 'top' of the camera. Flick it on at the 'top' and then reposition your hand into the grip. It is so easy. The key is to mount the strap so the camera hangs in portrait. I've seen lots of people diss the position of the power switch, myself included, but how many consider how it would work with the battery pack attached?
Edited to add: and now suddenly the position of the power switch on Canon DSLR's makes sense, too. With a battery pack installed on the bottom the power switch is right next to your thumb when you hold it in portrait. The 1D Canons and the battery grips all have strap mounts on the bottom, too. Shows you that I've never used a battery grip or 'pro' body before! The 7D has the power switch repositioned on beneath the mode wheel, but the Lock switch is now next to the thumb when holding it with a portrait grip.
Second edit: when you have the battery grip attached, you can insert the battery into the grip without having to have a battery in the camera. Super convenient.
Third edit: a very nice piece of design in the grip is the rubber covers for the contact terminals. The bottom of the OM-D and the two parts of the grip have contact terminals that communicate between grip and camera. These contacts are protected with rubber covers which must be removed to attach the grips. Each grip has a recessed area molded into them which hold the rubber covers, so if you need to remove either grip, you've got the rubber covers on hand to protect the terminals from dust and water. Very, very cool.