I've been spending a lot of time with the 50mm SMC-A f1.4 on the K-5, and also on an old MX body I recently picked up cheap on eBay. The lens is fantastic, but I'm finding that focusing it by eye wide open on the K-5 is very prone to errors.
On the MX it has an old fashioned split-circle thing in the middle, as do all the other old 35mm bodies I've used.
I came across this company on the web who make replacement focusing screens for DSLRs:Pentax K-5 & K-7 KatzEye Focusing Screen
Does anyone have any experience with one of these (positive or negative!) ... I can't think of a reason not to buy one, but maybe i'm missing something