New Years Resolution - actually to post pictures! So lets start with this pic of Guildford Cathedral for the competition. Taken with a Super A on Delta 400 (If I recall) using the K17 f4 and scanned using a Minolta scan Dual III

As I said it has modest but regular barrel distortion at 18mm - probably no worse than the 18-55 (I don't have one to check) and certainly not obvious in most 'real life' situations as the pictures above show (though in fact a close look at the windowframe on the left of the top picture shows it to be slightly curved)

I would say (from VERY limited personal experience and assorted reviews etc.) that it is on a par with or better then all the lenses you list except the 50-200 which, despite being a 'consumer' zoom is generally (IMHO rightly) regarded as a bit special. (the A 28-80 is not highly regarded, the 80-320 has had a mixed reception, the 18-55 is fine provided you don't want to use it wide open)

There was some discussion of this here:
https://www.pentaxuser.com/forum/viewtopic.php?t=5684&postdays=0&postorder=asc&highlight=sigma+hsm&start=15

Check that the focus mode switch on the camera is set to manual - otherwise it will only take a photo when the subject at the focus point is exactly in focus (useful under some circumstances but generally a pain).

Thought I might join in the fun...

AF Primes
F 28 2.8
FA 35 2.0
F 50 1.5
105 2.8 EX Macro

AF zooms
DA 16-45
DA 18-250
FA 28-105 3.2-4.5 (the current one - silver)
DA 50-200
Cosina 19-35
Sigma Apo Macro Super 70-300
Sigma Apo 170-500

MF Primes (I cut back on these a bit a few months back)
K 17 fisheye
M 28 2.8 (type 2)
M 50 1.7
A 50 2.8 Macro

MF zooms
A 35-70 3.5-4.5

I also have a 3rd party bellows and a Sigma 1.4x converter - amd the Sigam AML auxiliary lens that goes on the front of the 70-300 making it 1:1 at 300mm (but just try keeping it still...)

What do I use?
Digiltally:

Up till now if I go out with just 1 lens it has been the 16-45. That will become the 18-250 most times now

If I go with my small camera bag I add the 50-200 (or perhaps the 70-300 if I think I will need the reach), one of the macro lenses and one of the AF primes. Plus 360FGZ and filters

Rucksack - very much depends on destination but it gives room for the 170-500

AF film
One lens 28-105 (great lens!)
Small bag - add 19-35, 70-200 and a macro (probably the 105). Might drop the 19-35 for a prime at times

MF film - it is usually in as an 'auxiliary' with B&W film in. The 50 1.7 only comes off occasionally - usually for the K17

For the K10D use Pentax's own cleaning kit. This is a stick with a kind of tacky acrylic end which you press lightly agains the sensor which picks up any dust and then clean the stick of on one of the pack of tacky sheets provided. It works - I've used it. Very detailed instructions are provided and the translation into English is OK

Other dry cleaning methods should be OK - my first resort is a rocket blower and I only resourt to the sensor cleaning kit when that fails.

ON NO ACCOUNT USE FLUIDS unless you are absolutely certain they won't damage the special coating on the sensor - Pentax specifically warn about that somewhere...

Just for the record, I like prime lenses (I have a K17 fisheye, M28 2.8, F 28 2.8, FA 35 2.0, M 50 1.7, F 50 1.4, A50 2.8 Macro and a sigma 105 EX macro).

What do I use for serious photography? It depends (a) what the subject is abd (b) how important the convenience factor is.

Oddly arguably the best lens is my collection from an image quality point of view - the F 50 1.4 - is used mainly for (indoor people) snapshots, as I prefer not to use flash if I can avoid it. For 'serious' landscape photograpohy I use the 16-45 almost exclusively, backed up by either the 50-200 or my Sigma 70-300 for longer shots.

Just thought I would let you know that my Pentax version arrived today (Chris posted it last Monday I think but Italian post aint great).

Very first impressions
i) feels well built and operates smoothly
ii) smaller (when retracted) but heavier than my 16-45
iii) MF has that oddly 'grindy' feel designed into it, that i remember from the days when I used a Tamron 24-135 but that makes for a more positive 'feel'. Focusing ring is a decent size. Travel is short (about 60°)but MF is relatively easy to use.
iv) no ability to use MF in AF mode like other DA lenses. Also no little 'window' in the lens hood for ease of adjusting polarising filters.

A couple of quick test shots at 250mm at F8 (handheld, with SR, running at about 1/350) shows pretty good sharpness, but noticable CA. It is quite easy to eliminate the CA in the Pentax Lab software.

Looking through the viewfinder at 18mm suggests barrel distortion is modest and regular. Even the 16-45 is significantly less than perfect in this area.

Now I only need to take one lens when I return to England for Christmas!

AFIK lens SR can only correct for vertical and horizontal movement, whereas at least on the K10D there is also coreection for rotational movement, so in theory in-camera SR should be better.