Back in 2011, I bought the 18-135 WR zoom for my K-5, which was then the Pentax flagship. It was to be the only lens I took on a trip to Hawaii with my wife for our 50th anniversary. I figured the weather resistant feature would be great for Hawaii's rather constant rain, and the zoom range would cover me for most situations, with no need to change lenses and introduce the possibility of dust on the sensor. I made the perfect decision. Occasionally, I wished for more reach for distant subjects, but that was probably the case in only 5% or so of the shots I took. Later cropping in post pretty much made up for that problem. The lens is not the sharpest available (only the primes seem to meet that criterion), but still very good indeed. This lens is now mounted on my K-3 so I can "grab and go" for spur-of-the-moment shots, knowing that the chances are very good that I'll get a very acceptable shot. I leave the mode selector on "green" so if I don't have time to fine-tune the mode, it will still turn out well for "quick and dirty" shots where time is of the essence. Turns out it's now my most used lens, and the K-3's increased resolution over the K-5 materially enhances the quality of the results with it. Focusing with the K-3 is quick and quiet, and reliability has not been an issue even though I have one of the earlier issues of the lens. I have relegated my 18-55 WR to my K-5, which now serves as a backup unit.
For wider angle shots, I have the 15mm Limited, and for extremely crisp macro work, I have the 35mm Macro Limited. For low light shots, the FA 50mm f/1.4 works nicely, and for more reach, I have the now-out-of-print 18-250mm. So I'm pretty much covered, but that 18-135mm serves well as my "do all" lens. I highly recommend it for you. By the way, many of my Hawaiian shots with the lens are featured on the site below.
John
PENTAX : PENTAX Photo Gallery artist page