[davidmanze]

The 560mm also is not a very good lens... 

Typical of Pentax (to me) seems the bad lens designs of the newest lenses. Take that off 20-40mm f2.8-4 for instance. Why that limited range? Why the relatively small aperture (f4)?  It should be pretty darn special then, but it isn't at all. Fine in the center, nothing special in the borders. And then the price... Yes, it is weather sealed. But $1000 for a 30-60mm ff4.2-6 full frame equivalent? For that money Canon almost offers a 24-70mm f4 L IS USM.

 

Same goes for a lot of the lenses in the Pentax range. Not the best rendering optics, or steep pricing. Sure (and luckily), there are some gems in between also. 

 

With Canon and Nikon, photographers after shallow DOF possibilities are offered cameras with FF sensors and lenses with big max. apertures. For architectural and other purposes, Nikon and especially Canon offer a range of tilt shift lenses. And then there is that special Canon MP-E 65mm f2.8 macro. Not just with tele the Pentax range is limited.

 

For me it is a bit hard to pinpoint which kind of photographer a Pentax DSLR is the ideal choice for. I guess photographer looking for a basic "street" primes setup can find his way with the 21mm f3.2, 31mm f1.8 and 77mm f1.8. 

Agreed, in fact it's the middle period lenses that were probably the best, their recent lenses just don't raise the bar sufficiently and as you say DA 20-40 is decent but no more, needing stopping down to 5.6 to get decent sharpness across the frame and at a $1,000, however Pentaxians are seemingly very happy with it as they are with the  DA18-135 and the DA 55-300 both very average optically. That set against Sigma/Tamron who are standing toe to toe against each other and releasing a new level of class lenses gives Pentax a good reason for upping their game! 

  They were at their peak in the eighties with the FA* 300 F2.8, FA* 28-70 F 2.8 and the like, those were the days!