Don't be confused by the 35mm equivalent focal lengths. The lens gives the same magnification, but the digital sensor crops some of the full-frame image off, giving you less in the frame as captured by your K10D.
Now as for the FA* and 70-200 comparisons, believe me when I tell you this, I have agonised over the FA* for a while, and after having two opportunities to grab one I decided to pass both times - which made me feel a touch of remorse at each opportunity.
This is partly because of the reputation the FA* has but also its unrivalled build quality. I've held one once, but never got the chance to test it. I personally don't mind the weight but the IQ, even though it is absolutely excellent, is apparently not a great deal ahead of either 70-200 option available on the Pentax.
I would suggest that if you're considering the 70-200 by Sigma (the newer HSM version), then consider the Tamron that was released soon after the Sigma. It is just as good, if not better, than the Sigma in IQ and reasonably well built - both are heavy and large, but not more than the FA*. They are both plastic, though, whereas the FA* is metal.
If you have the opportunity to buy the older Sigma 70-200 EX DG version (not HSM), I would say this would top the bunch for reliability and IQ. Most people here vouch for this (contrary to what Sigma would like to promote about their HSM version).
So with this in mind, give it good thought - if you are happy to drop the funds, the FA* is a lifelong keeper, whereas the Tamron (or Sigma) 70-200 is better value with a downer on the build quality (which may be important to you in your situation).
Last edited by Ash; 10-25-2009 at 01:39 AM.