At the risk of being burnt at the stake (and I should quickly add that I'm fond of Pentax and love my 67), I see the K-5 and the Pentax lens line-up as being a purely subjective purchase.
Objectively, it's hard to rationalize. A number of other DSLRs will give equal IQ across the board, do a number of things better, and, with the exception of Sony, offer a broader choice of lenses. Not to mention all three main players offer the potential to upgrade to full-frame at some point.
I don't know that Pentax is in a position to offer full frame and compete with the other three because of price.
Still, I see the appeal. Just don't know I would go that route myself.