Steve Huff has posted a comparison of the Olympus E-P3 with the new 12mm lens against the Leica M9 with the Summilux 24mm lens. I remain terminally unimpressed with Leica. I see it as a toy for people who want to feel self-important. But using Leica as the bar is a great idea since, even though the cameras and lenses are all hilariously overpriced, the quality is certainly excellent.
A few interesting notes. First off, the Olympus 12mm represents itself wonderfully. It is sharper across the frame than the Leica at F2, but while the Leica would likely gain a huge boost from further stopping down, the Oly is performing around its best at this aperture.
Second off, the depth of field differences that make determining sharpness so difficult also provide a great example of the amazing difference between the 2x, Micro 4/3 sensor. The Leica shots have a great deal more texture and complexity, simply because more of them is blurred.
Third... off. The differences between even a bad full-format sensor, like the Leica, and the m4/3 sensor in the Olympus is quite large. One doesn't realize this until presented with full-resolution examples of real world shots. The noise levels out of the Olympus are quite noticeable, especially in the way that they negatively effect the rich colors in the sky.
And finally, I don't understand those who say they never encounter moire in their shots when using cameras without a low-pass filter. Now let it be known that I would always prefer a camera with no low-pass filter. I post-process every photo that I take and removing moire is easy. But still, every medium format picture I see has significant moire somewhere, and the first Leica shot, of the donation bins, has moire dead-center. Look at the sticker just above "RECYCLING CENTER." The letters are very discolored.
So the lens is great. And on an Olympus body, it focuses like crazy. But that THREE-YEAR-OLD sensor wasn't terribly good when it came out, and it's even worse now. And like I said, the Leica's sensor is the worst full-frame sensor on the market. As far as noise levels go at base, the Olympus looks even worse in comparison to the Pentax K5, Nikon D7000, and downright ugly in comparison to other full-frame cameras.
I love the lens, the sensor is needs to be ditched.