I was given a Pentax 50/1.7 for Christmas (I actually got someone's entire ME Super kit, which is great as that's my current favourite K-mount camera), and although I've only run a couple of rolls through with it, I'd say it's a bit sharper than the 50/2, but not enough to make me jump around and shout it's praises from the heavens. It's main advantage to me is it's somewhat shorter than the 50/2, which makes it easier to fit in a camera bag with lens attached
My favourite Pentax-compatible lens is still the M42-mount Jupiter-9 (85/2) with an adapter, which of course has its limitations - all manual, and tighter focal length. For walk-around shots I gravitate toward either the 50/1.7 (because I happen to have one - the 50/2 would work as well) or a 28mm I picked up recently. I also picked up an M42-mount Industar-50-2 (50/3.5) that's a pancake, and cheap - haven't tried it out yet, though.