Just ordered the Panny 100-300mm for birding
...after seeing the Oly 70-300 II annoucement.
From what I heard so far it is uncertain if one has better IQ over the other. But both are slow lenses so the half-a-stop advantage from Panasonic is more important to me than the size and weight benefit from the Olympus. Not only it will help on overcast situation, but the bokeh should be better too (which are important to wildlife shoot). Also, the Panny is cheaper (at $450 from Amazon's third party vendor).
I'm excited to try this lens on my OM-D. Although I do keep my expectation realistic. It is a consumer grade lens so it can't compete with my usual wildlife setup (5D3 + 500 f/4 or 100-400L). But hopefully I won't be too disappointed.
Will post some pictures (probably on squirrels) from both setup once I got the lens