The 100-300 ( 200-600 equivalent in 35mm ) Lumix zoom lens is pretty good. I think the weakness of the M4/3 systems for me was the AF did not match my 1 series Canon bodies. My GH3 was fine for big cats laying around while shooting from a Land Cruiser, but not really for birds in flight. A friend says the new Oly OM-D E1 will grab birds in flight; if true, that is quite significant.
The decision by Panasonic and Olympus, to incorporate lens error correction into the camera bodies themselves, means that the Lumix 100-300 zoom is pretty decent, small, light, and for $600 is very inexpensive compared to a Canon L 600mm lens. No lens corrections needed in Lightroom or Photoshop, since they are already corrected in software inside the camera body.
I shot this with an OM-D E-5 with the Lumix 100-300 -