http://www.photozone.de/pentax/597-pentax_18135_3556?start=1
It scores only 1.5 points out of a total of 5. And so does the price-to-performance ratio. :-o :-( The final verdict of Klaus is quoted as follows:-
"Verdict
We were quite enthusiastic about the Pentax DA-SMC 18-135mm f/3.5-5.6 ED AL [IF] WR at the time we received it. Regarding the high pricing of the lens we were expecting a very good performance throughout its broad zoom range and all that in a very compact, high quality body. Unfortunately the testing reality revealed a mediocre optical performance at best. A lens with a 7.5x zoom ratio may be quite ambitious but other manufacturers managed to design pretty good lenses with an even more extreme range. The Pentax lens is actually very good to even excellent in the image center but the borders/corners suffer from massive field curvature at the wide end and plain sofness at tele settings. This is certainly no issue for portraits and such but you don't really want use this lens for architecture or landscape photography here. This is also in so far surprising because the (cheaper) Pentax SMC-DA 18-250mm f/3.5-6.3 (a Tamron design) was actually very usable here. The secondary characteristics (distortions, vignetting, CAs) are about class-average. A very positive aspect is the build quality of the lens - the lens body is based on tightly assembled, high quality materials and the weather sealing is a quite unique selling point. The new DC AF motor may not be as fast as Pentax' SDM but it's quite fast and comparatively quiet. The biggest problem of the lens is its pricing which is simply not in line with the optical performance. Therefore: Avoid! Better consider the Pentax DA-SMC 17-70mm f/4 SDM instead which is a far better lens."
And, do see also this additional remark made by Klaus at the DPR forum. The reason for why the publish of this lens review is delayed has been talked about. The chance was given to Pentax but the lens was checked to be "within factory specs".
Previous Related Stories:-
PhotoZone DA 18-135 Test Results Will Not be Good (Very Possibly..)
Ned Bunnell's (Official) K-5 + DA 18-135 Samples