Well, this lens doesn't seem to be decentered, so I thought I'd set up a tripod and take some shots at mid distance in the garden and look at what we've got.
even at f4 it's pretty good in the middle of the frame, the corners however are quite soft, the edges less so.
By f5.6 the corners have improved, and the centre is at it's best. The corners aren't really perfect until f11, when the centre has lost a little, but still perfectly useable.
Same story really, except that wide open (f4) the soft area is both smaller than at 18mm, but softer. Improved at f5.6, but not completely sharp until f11
only slightly soft corners wide open (f4.5) by f5.6 it's fine, and improves again at f8. Centre sharpness is excellent.
Wide open (f4.5) corners and centre all seem good and very sharp
Wide open (f5.6) there is again some corner softness - this seems to be more to do with curvature of field than with actual softness. There is also some CA creeping into the equation - this is very easily fixed in Aperture with a touch of the red-cyan slider. Centre sharpness still excellent.
More corner softness - which doesn't really go away when you stop down, CA is also evident, but the centre is still quite useably sharp.
So, on the basis that we don't believe in miracles (do we?), and to put things into perspective I'd sum up the performance by saying that you can get really excellent performance from this lens up to around 90mm, but you should be prepared to stop down quite a way if you want sharp corners.
Over 90mm you will get some softening of the corners and also some CA, on the other hand, shots are quite useable if the CA is corrected, and assuming that you aren't expecting miracles. Stopping down doesn't help so much at the telephoto end.
It looks like a kit lens, but it certainly doesn't feel like one! lots of metal, it feels very solid on the K5 - a lovely combination. Worth mentioning that the K5 with this lens is a tad sorter than the A55 with the 16-80 lens, a little taller and wider - but they really are very comparable in size.
I'm prepared to find horrors, but my initial impression is that this lens is really as good as one is likely to get with a 7 times zoom. Suddenly the price doesn't seem quite so out of order!
all the best