With Pentax, there are not a whole lot of choices in production for a modern Pentax lens, especially for an all-around zoom. This 18-135mm is pretty much your best bet. Luckily, it performs decently well for its price. I have this lens and the 35mm F2.4 for really nice, leight-weight travel combination. The focal length of 18-135mm covers about 27 to 200mm 35mm film equivalent range, which is pretty good and covers many types of photos (landscape, portraits, group shots, and mild telephoto for wildlife). Plus the 1:4x magnification ratio means that you can also do product photography and close up of flowers. This lens has a little above average distortion at wide angle, so architecture photos are not going to be straight unless you post-process the image. The modern Pentax cameras can fix distortion in-camera too, but its slow. Vignetting, dark shaded corners, is about average..nothing I would concern myself with. Contrast is great a couple stops down from wide open. The color reponse of the lens is fantastic and typical of good Pentax glass. Its not too warm like vintage glass but great and vibrant nonetheless.
Sharpness. This is the subject everyone looks at. Pentax seemed to attempt keeping the longer focal length as sharp as possible. I feel they succeeded for the most part, at all focal length the lens performs very good a couple stops down from wide open. The CON is that the corners are very soft and blurry. I took a test shot of my carpet, pointing the camera straight down (using a bubble leveler to make sure), and at all focal lengths the corners are soft. Now, before one jumps ship, it is recommended that one read the Ken Rockwell article on the importance of sharpness. Most of the time your subjects are not at the borders of the picture, leaving the rest defocused anyway with bokeh, so most of the time when I compose my shots they are still very pleasing to my eye. And thats what matters. Only when you do critical work like close-up photography does a sharp flat plane work in your benefit.
For your money you get a lot of lens. Its lightweight because it doesnt need added elements for optical stabilization. It's weather sealed meaning it can handle mild rain with no problem (I wouldn't say torrential rain however). A feathered petal-hood is included and can be inverted backwards on the lens for storage. The focus ring is dampaned and quick-shift, meaning you can focus even with the autofocus switch on. Its located behind the zoom ring and has "soft" stops. The 62mm filter diameter is not so large that you must get expensive filters. The autofocus speed is not up to par to Canon's USM but it is about the same league as Pentax's SDM. Accuracy depends on what camera you have, but mine did well on my k-x. No back or front bias focus on my sample.
All in all, if you are looking for a one lens do-it-all, then this might be your best bet. If the longer focal length is not of concern, the older Sigma 17-70mm might tickle your fancy, plus its cheaper. Compared to the Pentax 18-55mm kit lens, though, it is way better and a good upgade. As with Pentax, if you want the best, you might want to look at their prime lenses.