I have to agree with LL that the subject matters a great deal, especially IMO with Pentax lenses.
They all tend towards bright rings around specular highlights (some more, some less) and they all have that annoying CA that puts green or purple halos in the bokeh. But in the right environments, they can do very well indeed.
(The Minolta 135 Smooth Transition Focus and the Nikon Defocus Control lenses provide something that I haven't seen in other lenses, so without ever owning them I really like the samples I've seen from them.)
I'm sure the 31/1.8 will come up in this conversation, so I'll post an example where it behaves well:
I happen to quite like the Super-Multi-Coated Takumar 135/2.5:
And the SMC Pentax-A 50/1.2 does a fine job in most cases (this one at f/2):
I like most of the longer than 28mm Pentax lenses for most subjects, though.
--Sean