Serious half truth's here. Yes, Pentax bodies are amazing (thought about changing from Nikon because of how great the K-10D was, just magical honestly; though not in the slightest because of weather resistance) but Pentax doesn't make any great fast lenses anymore (outside of their pancakes, which are expensive and anything but a deal, and as any serious photo guy knows it is you, and then the lens that makes the photo, not the body.)
Old fast Pentax lenses are disproportionally expensive and quite frankly not worth it, especially when you figure in the durability of Nikon's metal hotshoe flashes. New f4 Pentax lenses are inconsistent at best. Unfortunately they just don't compare... I wish they did.
Maybe I'm not that picky or just lucky... I've had one Pentax lens that I sent back but that was because the hood was cross threaded onto the lens and it had cracked. A new one was sent back to me and it's been fine since. That's the only defective lens I've received and I've bought at least a dozen lenses not including used ones. If you include used lenses, I've bought and sold several dozen from many sources including some Sigma, Tamron, Quantaray, Voigtlander, sears, vivitar, and others... all without a glaring issue.
As for cost... yes used lenses are nowhere as undervalued as they were 5 or 10 years ago. I wish I was a collector 10 years ago... I could have made a ton of money. But I say undervalued because these lenses are in demand now and people are paying what they think they are worth. You can still find some great deals in used stores - last summer my wife wanted to stop by a used store and I wandered over to their cameras and I saw a F50 1.7 for $50!!! That's a $200 lens! The great thing about Pentax is that all lenses throughout their history will meter on their current cameras. All m42 mount lenses, all k mount, all 645's... that's millions of lenses. If you can't find a deal on some of them somewhere you need to look harder. Even market value on a Helios 44 is an amazing deal... $50 for a zeiss copied lens (you do have to buy a m42 to k adapter for about $30 for the real Pentax ones). Here's some photos w/ this lens...
helios w/ ext tube [flickr.com]
helios 44 with ext tube [flickr.com]