There is certainly a difference with Pentax lenses over any other SLR legacy lens option.
To me that difference is 'balance'.
A fine balance in size, color, flare tolerance, weight, build, performance, bokeh, price.
Every lens is a trade-off in each aspect; size for sharpness; sharpness for bokeh; etc
But the Pentax lenses give me the feeling that it trades off each aspect for what the lens would normally be used for.
(eg. A 20mm being small and just need a f4 rather than a f2.8 for landscape use)
I have other brand lenses too which I often try to buy a set of (eg. FD 20/2.8; 55/1.2; 100/2; 135/2; M42; M mount)
But its the joy of use and this 'balance' that makes me reach for the Pentax lenses more often.
Yes, other lenses can be sharper, less distortion, faster, etc.
But I often find the trade-offs too big for one aspect against another.
As mentioned by another poster, you'd need to buy a large number of the Pentax lenses since many are good.
My own favorites :
M20/4 - small, portable, nice color, good performance except a bit on the corners (which I think is a fair trade-off); I use it more often than the technically better but larger, heavier, more expensive and less flare tolerant Voightlander 21/1.8 ultron that I have)
K28/3.5 - almost like a 50mm prime f5.6-f8 type optical performance; (and its 28mm not a easy to make 50mm)
FA35/2 - Very nice all rounder lens. Easily rivals any 35mm out there (and usually for a bit smaller size )
FA31/1.8ltd - Another versatile lens; Well worth it on FF. Typical Pentax drawing style and right contrast.
K50/1.2 - One of the best f1.2 50mms out there. IMO, its the one with the most situations where the bokeh looks good. Yes, I'm also claiming that wrt to the so called 'king' rokkor 58/1.2 (which in many situations can have horrible bokeh as much as it can have good ones).
A50/1.4 (or FA50/1.4) - A do it all 50mm for not much money and again, more consistent bokeh in most situations.
FA77ltd - a special lens. Period!
M85/2 - CA galore in the wrong conditions, but a nice drawing of the subject, rich colors in many others. Super compact, the size of a 50mm. Simply a joy to use and take out as its so portable.
K85/1.8 - the best of the slower 85mm. Almost w/o weakness. Sharp, fast at f1.8, not too big unlike f1.4 ones.
FA85/1.4 - one of the best f1.4 85mm out there.
K135/2.5 - very value for money 135mm. Rich colors and nice bokeh in more situations than the highly acclaimed FD135/2 that I have imo.
There are others, but I think this list should be long enough...
Because it covers A7 and other brand lenses, I don't post another thread that I have on another forum here, where I give my impressions of the lenses I use :
http://www.clubsnap.com/forums/showthread.php?t=1488003