First off I own 2 Oly E-M1's and love them both, one Black and one Silver... I now own and use all 5 of the new Oly MFT Pro Lens Line-Up, plus the Oly FT Pro 300mm f2.8 Tele Lens. Secondly, I am a Nikon Pro equipment user for over 53 years. Olympus has won me over and everything else is gathering dust. Now for the New MFT Pro 300mm f4 lens. It is a beauty, hands down. After using it a bit and having it break-me-in, LOL, It is outstandingly sharp, IS in combination with the E-M1, hand held is to die for! I am mostly a action sports shooter, Football, Basketball, Lacrosse, Water Polo, Surfing, Track and Baseball. The only down side is the f4 limitation with this great Pro 300mm f4 lens and having to crank up the ISO etc. in low light or poor stadium indoor/outdoor lighting. For bright daylight, this lens will get the ACTION job done but not so in low-light, I leave it at home and use my Killer, Stop Action Olympus FT 300mm f2.8, with the MMF-3 and the EE-1 Dot Sight! This combo gets "IT DONE!". My other E-M1 is paired up with my Pro 40-150mm f2.8.
The only other gripe I have, so to speak, is the lens hood! Olympus has a great idea and they should have used the same concept as the have on my MFT Pro 40-150mm f2.8!! I am looking to remove the hood and find a screw-on hood to replace this binder hood. "It does lock-up and almost impossible to retract. Aside from low-light limitations (FOR ACTION ONLY) and the poor hood design, this lens is a star. I am writing a request to Olympus Corp in Japan and asking them to consider building a MFT Pro 300mm f2.8 or f2.0 lens with all the features my little jewel (Pro MFT 300mm f4) has, less the existing hood design. Now that would be a dream lens for sure.
In closing, overall, this is a great lens to have and yes, I would get it again after owning this one and I do recommend getting it...! Build quality is spot on, "Top Cabin" and exceeds my older FT Pro 300mm f2.8 lens, Kind-a-Maybe!
V/R The PhotogDog