[Ayoh]

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!  

Good point about the 20-40mm, that lens is a mystery - slow, expensive, soft etc. The appeal is the nice build I guess, but that's where it ends.

 

As far as the 560mm is concerned, it is quite sharp from what I have seen so it is a good lens, just quite overpiced. 

 

The appeal of the other limited lenses is also there I think. They have good (if not exceptional) optics with a unique build and small size that has Leica type charm without the price. Handling your standard plastic lens seems rather dull in comparison. These characteristics may not appeal to everyone though.