I started with Pentax in high school shooting for the yearbook using the Pentax KM that was my grandmother's retirement gift. She worked for Honeywell / Pentax in Englewood Colorado in the seventies. The KM was later given to me as my high school graduation gift! I'm about to turn fifty and I've literally been a Pentax shooter this entire time, without exceptions. For years I had only three lenses: Pentax 28mm f/2.8, Pentax 50mm f/4 Macro, and Pentax 200mm f/4 Telephoto. It was a great combination. I later added a 20mm, 24mm, 85mm, 70-210 zoom and a 300mm f/4 an MX and an LX. I stuck with the Pentax lenses because the quality is truly there and they can be trusted. I can say they've Never let me down. I appreciate that Pentax does not change their mounts and I can use my early eighties vintage lenses on my new K-5. (My Canon friends are out of luck, unless they want to use their old bodies.)
My only exception to the Pentax Lens rule is a recent purchase of a used Tele Vue Pronto Telescope 480mm and a 2x Powermate. I was happy to have this Telescope during the 2012 Annular eclipse and the Venus Transit. I shot the Transit successfully from the summit of Vail Pass with my LX and Fuji Provia 100 slide film and of course, a solar filter over the end of the telescope.