If you're gonna buy a Pentax 50mm f1.2 go for the A version, that way your can use it easily on your K5 or K3, where it makes a magnificent portrait lens, not to mention blowing away the Canikon folk who dare to question your choice of camera. Of course, focusing can be a problem. I installed a Katzeye screen in my K 10 which helps a lot; I haven't used it enough in my K 5 to have a good opinion. In my LX, MX, or ME Super there is no problem. Just magnificent! You do need to be veeeery careful, however!
Oh, and did I mention the reactions from the great unwashed when you casually mention, "Oh, its a fast 50, an f1.2." Simply priceless!