Here is gpwhite at the Leica forums, who has one:
"Well, my Q does not achieve sharp corners, and it shows rather weak corners. PERIOD. But the more I shoot it, the more I am engaged and enthused about my Q.
For me, the Q @ f/2 (seems it nicest opening) is at a large gap below the 28 Summilux @ f/2 on M. Yet, the Q is biting sharp and very good 3D... there seems to be field curvature or something behind the curtain with the Q. It is not pleasant.
My hunch is that the image you posted, chrismuc, was somehow brought forward in the processing stream without in-camera distortion correction (software/ glitch). I shot a couple of tall buildings yesterday, and the lines are perfectly correctly. I not percieve any curvilinear distortion (on a monitor, as prints are a few days off).
IMHO, IQ at distant focus on the Q, however, does not rival the laser-etched, straight lines of the 28 Summicron (at say 10m). I have not shot at a distance with the 28 SX yet.
That's my download so far..."
Very refreshing, honest report.