ePhotozine tested the OM 12-40mm lens and concluded:
There is little sign here that the smaller MFT format is causing much in the way of diffraction issues with this lens. Sharpness is maintained throughout the aperture range and also throughout the focal length range, to an extraordinary degree. There have been many claims in the past that zoom lenses perform as well as any primes within their range, and in this case that is definitely true.
The cost of this is a slightly higher price and an increase in bulk, but the latter point may be moot as the lens is not particularly heavy anyway. We really see the benefits of the compact nature of the MFT format here. It is just possible that some might choose the more compact 12-45mm f/4 option and save £300 in the process, but then again we have to set this against the more expensive lens that has a truly outstanding performance level.
It can definitely be Highly Recommended.