Having owned all three, the G5 feels like a G3 with a slightly curvier GH2 grip. I had thought that they should have just put a GX1 grip on the G3, but the G5's grip is far more positive. Still a little tough to hold a 14-140mm one-handed, mostly due to the short height of the grip giving the pinky nothing to rest on. However, that's a really heavy lens for this system, and anything short of that is pretty stable one-handed.
Like all Panasonics with built-in EVFs, the eye piece still sticks out too far from the back for my taste. I find that is makes it too hard to place in a jacket pocket. The E-M5's eye-piece sticks out less, and then they offer the option of a larger one.
With the e-shutter and flip screen, it can make a really great waist-level street cam with the 20mm. Cradle it in your left hand, thumb and forefinger around the lens, and flip the screen out so that it is sitting on (behind, for anyone looking at you) your wrist turned up toward your face. The right thumb can then be used to subtly press the shutter. It just looks like you're carrying it around in your left hand. Nobody who isn't watching you closely would ever notice that you're actually taking pictures.