This lens is a revelation for me, having recently swapped out my Canon system over to a Pentax K-5 (mainly for budget reasons) and a lens like this is a strong argument for staying in the Pentax camp.
I believe this lens represents extremely good value for money, the first and most obvious thing when you pick it up is the build quality - it's outstanding. There's a real classical zeiss/leica rangefinder lens feel to the lens, it's so far ahead of the plastic kit lenses, you'd be forgiven for thinking it costs over a grand. The action of the manual focus is excellent and well damped, although this lens is hard to manually focus at distance, it's biased towards the macro end. It features a built in, pull out hood, which precludes the use of Polarisers when in use but in practice flare is well handled with or without the hood.
Optically, the lens performs above it's price bracket, but only just: It's a macro, so it's not as fast as other 35mm primes, but it is very sharp from wide open onwards, something you can't say about many lenses. Colour reproduction and contrast are also very good. Background blur is good, but not exceptional, it can be a little busy, rather than super smooth like the best lenses, but it's certainly not bad at all. Focus is fast, less noisy than other Pentax lenses, but can a pain when it hunts, the throw is long, again because of the macro. A limit switch would be nice.
Macro performance is very good, a proper 1:1, although working distance is TINY at that range so you need to be aware of that. I would personally say this is an excellent walkaround lens, especially on bright days. The optics are great, it's light, well built and lets you get in close when you need to. It is not a low light solution, if you treat it as one, you'll be frustrated.
I wouldn't hesitate to recommend this lens to others and I can see a Limited lens addiction in the pipeline.