Quote: If you aren't going to shoot Pentax glass, you've pretty much eliminated any reason to use their cameras.
Humm.. What about SR, weather resistance, Pentax colours, great body designs, great viewfinders, for starters?
To answer the original question, I have one lens that's not a Pentax. It's the Sigma 17-70. And it's my most used lens. I confess I try to look at Pentax first when selecting a lens, because I know I won't be disappointed, but a good lens made by anyone is a good lens. And the Siggy is an amazing lens.
When I bought my DA21, I could have gone with some of the wide Sigma primes (20, 30 mm etc) but I wanted, among other things, the small size of the DA21. Otherwise I would have gone with the best optically, whatever the brand.