I think, with reading the various replies here, is go with what you feel is best, and what you want to get out of the image.

If the image "feels" right, then it is right.

The main rule of thumb is that no wider than a 50mm on APS-C (75mm eqv) for Head and shoulders shots.
Around 35mm (around 50mm in FF) for full length shots.
Any closer, not a problem - to an extent, probably around 200mm in full frame.
Any wider than 35mm then you get "no ears" and elongated noses.

Of course there are exceptions to this rule

Bad news I'm afraid Stefan, Sigma don't do a 17-50mm

They have, according to their web site, the following

17-70 DC F2.8-4 Macro
17-70 DC F2.8-4.5 OS Macro HSM
18-50 DC F2.8-4.5 OS HSM
18-50 DC F2.8 EX Macro

Reading the specs, I think the DA* 16-50mm is looking more and more the option.

I know what you mean, the upside is that we may start getting more and more lenses that we want ("new" 85mm, 135mm, 70-200mm, 30mm F1.4, 400mm 500mm etc etc)
Downside, we get CaNikon riff-raff converting to Pentax and second hand lens prices go through the roof (oops that second bit is already happening...)

A tough choice. Although I like Sigma EX series lenses these days, they are well built and reliable - not like the none EX Sigmas - they are also relatively well priced for what they can do.
The 16-50 however is a different matter. Early examples were hit and miss on their quality, but having said that, in the last 2 years I have not heard of a faulty one at all, maybe Pentax have ironed out the faults with the increase in production.

The three big advantages of the Pentax over the Sigma are the extra 1mm, and at that wide angle does make a difference, The fact that it still has the screw drive for none SDM equipped bodies (so you can use it with the ist series) and thirdly, probably the most important, it is weather-sealed, and this is a boon, as you will know, having the 50-135.

So for me, I would go for the Pentax, but the Sigma would be a fine alternative.

No, its not a link to a webstite acclaiming Pentax finally, but the numbers of Pentax users you now see around.

I was in Dubrovnik last weekend, and obviously took the camera. In past years I would by the only person carrying a Pentax around, amongst the CaNikons of this world, however this time, I saw at least a dozen Pentax DSLR's all over the old town. Sure the CaNikons were a plenty, but it is indeed good news that more and more people are choosing the "right" camera...
In case you are wondering about the others, well two Sony's and one Olympus DSLR were around.

Also, my friend (a Canon User i'm afraid) was in the USA, specifically Yosemite over the last couple of weeks, and it was the same there. The usual CaNikons, but also a great number of proud Pentaxians too.

So, all, time to Rejoice, the word is getting out.

Ditto, I only use ND grads and a polorisor as my filters on my DSLR.
I do have some old A series Cokins such as blue grad, tobacco grad, etc but only use them very rarely.

Keep to a polorizer and an ND grad (or a range of them) and you can't go wrong.

I have the 24-70mm (the MK1 EX DG Macro), and it is indeed an excellent lens, built like a tank, sharp, good colour rendition, but HEAVY! Use it with a battery grip to balance it out on the body.
Downside, not wide enough on APS-C (36mm eqv) however it counters by being nice and useful at the long end. Compliments my 16-45 and 50-135 well, and useful as a portrait lens too.
Worth a serious look at.

I have both the Tamron 90mm (OLD SP 2.5 1:2) and the Sigma 105mm.
Both I can say stunning! I have not used the Pentax 100mm, but if my two are anything to go by, it will be superb.
Any of the three will be worth having on your kit bag.

I think John has got it about right. The 85mm is probably the most useful portrait lens, many Pro's swear by that length. However, I have managed to get some great images from my, 50mm, 55mm, 90mm Macro, 105mm Macro and my fave portrait lens, the mighty 50-135mm. However I feel that 135mm is still a great length especially for just face shots. Now I know it is EFOV of 202mm, it is still however a 135mm lens. I've not heard that much about the "tele-effect", but although it is a short tele it is still very useful, and probably still short enough not to have that flattening effect.
An example is here, which was at 135mm on my K20D.
It helps having light to suit, maybe to reduce the flatness.

Just looked at the Sigma Web site and the Bigma does show HSM and OS for Pentax Mount. Even going on to say that AF will not work with none SDM pentax bodies. So you could say it is already there.

I am more interested in Sigma making the 120-300 F2.8 for Pentax. Now THAT would be a great (albeit expensive) lens to have.