I recently purchased the Olympus 12-40mm f/2.8 lens for $800 (on sale right now) and discovered that it was sharp all the way to the corners, at all focal lengths, and free from purple fringing. See my conclusion here.
I looked through the sample photos at dpreview.com for the Leica Vario-Elmar-T 18-56mm f/3.5-5.6 which is the normal zoom for its T and TL series of APS-C cameras, and sells for a very expensive $1650.
Looking at some of the sample photos, I saw corners that were mushy and had purple fringing. Unlike the sharper and purple-fringe-free corners of the Olympus zoom.
Furthermore, the Olympus zoom is a constant f/2.8, and although that’s equivalent to a constant f/3.7 on an APS-C camera with a 1.5 focal-length modifier, it’s still a faster lens than the Leica zoom.
Furthermore, the Olympus zooms out to a slightly wider field of view.
The only advantage of the Leica lens is that it weighs less, approximately 9 ounces vs 13.5 ounces. But the Olympus lens is faster, and faster glass is heavier, that’s just the way it is.
Despite Leica’s reputation for making the “best” lenses, it seems to me that the Olympus lens is superior to the Leica lens. And that’s before we even bring price into it.