I've been contemplating the 50A for a while now and am also a Canon 50 1.4 owner. I think I can finally put the GAS to rest about this and will keep the Canon version.
Yeah. It took me 3 copies of the 35A to finally get a good one. But even if I was willing to go through that again with the 50A (I'm not; I'd rather spend my limited time taking pictures.) I don't think I would because of the size/weight.
Why is this lens so big and heavy, by the way? Pentax and other lens manufacturers have shown it's possible to make compact lenses with very good IQ. I don't know anything about lens design, so forgive my ignorance.
If you are referring to Pentax's limited lenses, you have to look a little deeper at them to understand the trade offs. Ie: they have no built in AF motors (body driven- loud and clunky), and some of their apertures aren't as big.