10 out of 10 points and recommendedSuperb retro metal build quality and design. Weathersealing. Small and lightweight. Excellent all round optical performance. Decent close focusing ability. Prime lens quality bokeh throughout. Superb flare resistance.Not cheap - but you get what you pay for.
The DA 20-40mm is a very unique lens in my experience, a bit of a throwback to film days, and as such needs to be appreciated with that in mind. If you're looking for a lens with biting sharpness then this isn't it, though it is decently sharp all the way through. Optically is very good IMHO, minimal CA and distortion and sharp across the frame stopped down a bit. Colour and contrast are also very good and typically Pentax. Yes it has a short range, but you need to consider this lens as a combination of certain classic prime lens focal lengths. Close enough to 28mm all the way through to 60mm (35mm Equiv). So the classic focal lengths of 35mm and 50mm are also covered, because this lens does perform like a classic prime lens in terms of rendering.reviewed June 18th, 2017 (purchased for $574)
One area which the IR review completely missed is the Bokeh, or out of focus areas this lens renders, it's simply beautiful, far and away better than any zoom lens I've ever used. It has nine aperture blades which ensures that out of focus areas simply melt away in just the same way as a prime lens, at all focal lengths. Ok, because it's an F2.8 to F4 lens it's not going to smash the background into nothingness, but if you get close to your subject you can get some lovely subtle and smooth bokeh. If you like how old school 50mm prime lenses render then you will love this lens, it has that feel to it.
It is also remarkably flare resistance and the front element is very easy to keep clean thanks to Pentax's excellent Super Protect coating, which really does work.
Lastly, the price - I got mine on a special deal, saving myself a couple of hundred pounds, would I have bought it otherwise? maybe not, but now owning one I understand why it's not cheap and can appreciate the value of it. I'm really pleased I bought it and it's my go to zoom lens on my K3II. It also makes a great combination with the 15mm F4 and 70mm F2.8.
In summary, if you're just all about tack sharpness, it's probably not for you, but if you appreciate a lens for more subtle qualities, such as bokeh, flare resistance, colour, contrast and aesthetics, you won't find anything like it anywhere else, and for that reason it's a 10 for me. I've owned the Sigma 18-35mm F1.8 but I'd take this lens over that one all day long, in fact I already have.