ePhotozine tested the new Olympus 12-45mm f/4.0 PRO lens and concluded:
The standard zoom has much merit, saving us changing lenses constantly and offering a versatile set of focal lengths. The focal length is a vital aspect and here the usefulness of have that ultra-wide 12mm (24mm-equivalent) cannot be underestimated. The 45mm long end (90mm equivalent) is an ideal portrait length, although of course, being MFT format, we might wish for a much wider aperture to obtain the best out of focus background effects. The very close focusing is an added bonus.
However, the sacrifice for compactness and a lower price is that maximum aperture, and f/4 is really quite modest. There are plenty of f/4 lenses on the market and size, weight and price are the usual benefits.
Depth of field is an issue, but as well as a disadvantage for perhaps portraiture, equally well this could be an advantage for other types of photography. Perhaps close sports, groups of people, landscape and travel, in general, will benefit from more extended zones of focus.
The quality is there if we choose our apertures, and is definitely there in terms of distortion, flare and CA. The lens is very suitable for architectural shots, again perhaps coming under the umbrella of being a travel lens.