Started off in film with a Ricoh KR 10 and a Pentax or Sigma lens....think it was a 28-80. Added a Sigma 100-300 .
Then along came digital.
Non Pentax lenses today are:
Tamron 10 - 24. Ok. Have used it a bit with OK results, but its a lens I will often leave out of the bag and not miss.
Tokina ATX Pro 20-35 f2.8. an excellent lens, beautifully constructed, use it a lot. Purchased it in Hong Kong, standing in a Nikon store waiting for someone else and spied it at the bottom of a very big pile of boxes...asked about it, he said he had no idea what it was doing there or how it got there and to make him an offer, which I did, a very silly offer, he initially refused but then stopped me at the door and accepted it. Have used it on K1 mk2 and very
very happy.
Sigma EX DG 24mm f1.8. careful handling it, could cut yourself its so sharp. Looking forward to using this on the K1.
Sigma EX DG 70-200 f2.8. Has had a trip to Japan to meet its family at the Sigma factory, now K1 compatible. Again....a lens I look forward to on K1.
Tamron 28-300. Picked this up s/h recently, have field tested it and pleasantly surprised at results. My "lazy" lens for the K1.
Sigma 135 - 400. Haven't used this on K1 yet, but on APSC it is great up to 300mm and then gets soft.
Bought my wife a Sigma 18-250 for her K3, in addition to DA 17-70...she doesn't like it, says its soft. I have tried it and in good light you get good results, but if conditions are anything less than perfect you have to start to "work" it.
Overall my experience with third party lenses is very positive.