Haha, this lens. Let me rephrase that review for you:
Cons:
- Doesn't perform at 20mm until f/4
- Doesn't perform at 40mm until f/8 (just lol)
- Even then, strong vignetting
- Low contrast at 40mm
- With corners that never get sharp ever
- All this over a very limited zoom range
- With a very modest, non-constant aperture
Pros:
You realize that even ignoring real pro-quality first party zooms like the Canon 24-70 f/2.8 and the Canon 24-105 f/4L, which can be bought for roughly $880 and $650 used, you could outright buy new third-party zooms like the Sigma 18-35 f/1.8 or Sigma 24-70 f/2.8?
I get that the build quality is nice (so are its competitors), I get that it is one of the first Pentax-branded lenses to have USM-type focusing (all its competitors did long ago, even third parties), I get that it's compact (use a prime instead, it's not like the 40mm end is any good anyway), but none of that matters if it can't deliver optically. Frankly this sounds a lot like a kit lens in a lot of ways, and it just doesn't stand up to the competition in its price range.
"It's a difficult lens to summarize" is a copout, it's a limited-range zoom lens with only one length fully usable, and it's a mediocre performer even there, for the kind of money that should buy actual pro-quality glass. Buy a 21mm and crop your shot, I don't think you'll be missing anything.