Upon renting the EM-1 Mark II this year with the 40-150 f/2.8 and TC 1.4, I decided to buy. At first the menus almost made me violent, but after reading a PDF version of the manual on an international flight three times, given the C-AF and other capabilities detailed in this thread, this camera body along with certain lens combinations provides a premiere birding / nature system. I love the 40-150 f/2.8 combined w/the TC 1.4, it provides a stunning equivalent 112-420mm, and remarkably does it for under $2,000. Putting together a proper lens array will not cost the price of a new Honda Accord with this system. With similar enthusiasm I regard the potential of the 300mm f/4, but it is back ordered so I can only comment on that with anticipation.
So far B&H has sent me the body, the 12-40 Pro, 40-150 Pro, and the 1.4 TC. A separately ordered 300mm f/4 remains backordered well in to March. With accessories I'll ultimately be in for around $7,000; or, the price of one 7.5 lbs. Nikon AF-S NIKKOR 200-400mm f/4G ED VR II.
Shoot what gives you the images you love!