I had an SMC Tak 50/2 M that I used on the original NEX 5. Tiny little thing. It weighed less than the adapter. Good sharp lens with nice colors and decent bokeh. though. CA and flare control weren't the best, but for the money ($37 bucks for a really clean copy, EX+ by KEH standards) it was hard to fault.
i'm using a few Zeiss ZK 50 MP and 25 2.8 and Rokinon 85 1.4 lenses in PK mount with a tilt adapter on the NEX-5n. i haven't tried the Pentax K5 kit lens which is the only Pentax lens i have. i love being able to play with tilt on all the lenses i have for the K5.
They make two different adapters for pentax. One has a built in mechanism on the adapter that allows you to change the internal aperture controls on say DA lenses where they don't have an aperture ring externally and then another cheaper one that only mates the K mount to the body which will only work for FA and older lenses that have external aperture rings. I paid $25 for the one that does both through Rainbow Imaging. I actually still have it and sold all my Nex and pentax gear if you want it. Just pay post and it's yours...
