the 30/1.4 is an excellent lens. i considered switching to the 35/2... i actually have both at the moment, but i'm 99% certain i'll stick with the 30/1.4. the 35/2 has 3 advantages really... it's smaller, it uses (the pentax standard) 49mm filters, and it has a shorter minimum focus distance, allowing for more dramatic perspective when close focusing. because i usually keep the normal prime attached to my camera body, the compactness really doesn't help any... either way the body will still occupy the same location in my bag. as for the minimum focusing distance, it just wasn't worth the 2/3-stop tradeoff for me. the 62mm filter on the sigma really isn't that much bigger. for a polarizer, adapter rings aren't the end of the world.
i think i've just gotten accustomed to the convenience of f/1.4 in low light and to the handling of the sigma. i used it previously with canon and minolta, so it's the constant thread through my various camera misadventures so far. but i wouldn't keep buying it if it didn't deliver such great results.
i know you said you've already seen samples... there are a bunch more in the
sigma 30/1.4 group at flickr if you're interested.
the one thing to keep in mind is sample variation. my canon and minolta 30/1.4 lenses need to go back to the factory for calibration before they became great performers. the process is painless. door-to-door turnaround is typically less than 2 weeks, and the lens will come back functioning perfectly... but that's still too much of an inconvenience for some people.
to more specifically answer your questions:
it is not too heavy. i find it balances well on the K10D.
i like the bokeh.
focus speed isn't noticeably different from pentax primes i own.
i haven't noticed any problems with color. i do a fair amount of post-processing on a lot of my photos, but the starting point is usually pretty good.
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