That lens will work perfectly on the K10D, including AF. All the FA lenses will.

I have the SMC-FA 50mm F1.7 and it's a very good lens - the slight extra speed of the F1.4 you're looking at will be useful for portrait use when you want to limit the depth of field.

This is what photozone has to say on distortions:

Pentax

The Pentax DA 12-24mm shows a pretty good distortion characteristic for an ultra-wide zoom lens. At 12mm there's a relatively pronounced degree of slightly wavy barrel distortions (~2.1%) which decrease continuously when zooming towards the long end of the zoom range where the lens is basically free of distortions.

Sigma

The Sigma AF 10-20mm EX DC showed a rather unusual distortion characteristic. At 10mm most of the image field is almost free of distortions so the measured distortions figures look fairly fine here (taken a little inward from the borders). However, this is only part of the truth. If you check the 10mm distortion chart below you will notice that the extreme corners are actually quite (barrel-)distorted - probably in the 2-3% range. At 14mm the lens shows a moderate degree of pincushion distortions, less so at 20mm.

So it doesn't look like there is all that much in it, especially given the the comparison is between 10mm in one case and 12mm in the other.

The Pentax tests a little better for vignetting, and MTF figures are fairly similar (and both quite good) and for CA the Sigma tests much better.

Overall they don't seem to thing there was much in it between the two lenses so as the Sigma is significantly cheaper and significantly wider that makes it an easy choice for poor people like me, however I'm certain I'd be quite happy with either lens.

Not for me - if the Pentax was under £350 I'd have considered it however at £400+ the extra £110+ isn't that far off the price of a new FA 50mm F1.4

My motion entry was also done using the Sigma 10-20, in fact both my successes in the competitions so far were with that lens.

I'd also agree the lack of CA with the Sigma is noteable - my two most commonly used lenses are the 10-20 and the Pentax 16-45. If I shoot at say 18mm and can't recall which lens I used there is an easy test: if there is any CA then it was the 16-45, if none then it was the 10-20.

Sharpness and colour rendition is also very good. While it's possible that the Pentax 12-24 might be better (although I'm not convinced it is) but IMHO it'd have to work miracles to make up for the price difference against the Sigma (£629 v £289 at warehouseexpress).

I'm pretty sure it was.

It's not the sort of shot I take much of, however I've found the Sigma pretty well corrected for a lens that wide. You can get quite unusual results with people just due to perspective though. This one of my wife on the trampoline in our garden being an example:

It is indeed.

I've also used Cokin-P filters with the lens however like you I've also had problems below 12mm or so. I've got a cut-down holder somewhere (that only holds 1 filter) that'll probably work fine down to 10mm but I've yet to try it on this lens.

I use a polariser regularily on my Sigma 10-20 and have not had any problems with vignetting:

Either the Sigma 10-20 or Pentax 12-24 would be great options.

In the UK the Sigma is half the price of the Pentax which makes the choice a "no-brainer" for me, however elsewhere (especially in the US) the price differences seem to be considerably slimmer so that could be a big factor.

If both lenses were the same price I'd possibly have went for the Pentax - even though it's not as wide it'd have better re-sale value once Pentax bring out a full-frame body and I can start using my 20-35 F2.8 again. I wouldn't pay much more for the Pentax though.

I'm on the rangefinder forum (I have a Zorki 4K and a couple of other RF cameras) and the discussions on the M8 are quite interesting. Most of the owners seem reasonably happy with it now, however quite a few regard the Epson RD-1 as being better hence that camera now commanding serious prices used.

I rather like the idea of a digital interchangeable lens rangefinder however £3K is silly money for a camera as limited & problematic (there are still serious problems with the white balance) as the M8 is (compare it to the similar priced Nikon D3 for example) so, like many people, I'm hoping Cosina will do something under the Voigtlander banner although it doesn't look like happening anytime soon.