As a 15 month owner of a K100D I can answer some of your questions.  I've
read quite a bit about the K10D as well and seen many images.

First off, don't buy the K10D or any of the other 10MP cameras for reasons
of image quality alone as the K100D is the match of them despite being 6MP.
Up-res a K100D picture to 10MP and there's a miniscule difference,
absolutely tiny, and I would go so far as to say that using RAW and Silkypix
3 as a converter it produces the most natural colours out of any of the 10MP
and below cameras.  JPGs are fine but I find that the main problem is that
there is a fine line between perfect images and blowing out things like the
sky, and to get great results you will have the learn how to get spot-on
exposures, but the highlight flasher and digital preview helps a lot with
this.  RAW just gives more leeway and I find it easier to work with.  JPGs
also put more reliance on the white balance being correct at the time of
shooting, the K100D is fine I find in daylight conditions outdoors and
indoors, but in artificial light just be prepared to perform a manual WB
ritual first (quite easy), I don't find any of the presets much use with the
wide variety of lighting available these days.

To be worth going to the dearer cameras you have to be lusting after speed,
as that's all the K100D is lacking really.  The small buffer means it's not
a camera for rapid fire bursts.  Autofocus is fine and fast with the right
lens in good to dull light but below a certain level (pretty dim) it slows
down more than most DSLRs.  A flashgun with an AF assist light brings the
speed back up indoors though (even if flash isn't used), I use a Sigma EF500
DG Super which can shine its assist light without firing the flash in its
ISO mode.  There's no wireless flash, although I can use the built-in flash
to trigger my Sigma in manual mode set to a fixed power, so some fun with
remote flash can be had on the cheap.

I get about 600 shots from a charged set of Uniross Hybrios over any period
of time they're ever likely to stay in the camera (months).  These and the
Sanyo Eneloops are the K100D batteries of choice for rechargeables.  For
non-rechargeables, lithiums like Energizer e2's are the way to go (about
1500 shots).  Ordinary NiMh rechargeables are OK if they're good ones and
used up within say a week where good ones will get   about 800 shots, but
the low self discharge ones mentioned are much better in the K100D for
general use.  Don't use alkalines apart from emergencies (about 100 shots if
you're lucky) or rechargeable lithiums unless you're absolutely sure they
are regulated to 3v per double-pack (I forget the proper name) and aren't
3.7v each.  The K100D is a joy to use with old cheap manual lenses, the
camera gives an AF confirmation when manually focusing, and stop-down
metering is easy and works well (one extra button push required).

The K10D is a fine camera, it produces the best RAW out of all the 10MP
cameras mainly in highlight detail.  If you want sharp JPGs you will have to
change the default settings, a quick comparison with the competition may
show JPGs as having soft edges but there's better texture detail especially
at higher ISOs and it tends to look very good in print being this way.  Edge
detail can be enhanced by using Bright mode and reducing saturation.  It's
faster than the K100D and matches the competition for speed again until the
light is very low where it slows down more than Canon and Nikon, don't know
about Olympus.  If you bought a K10D it would be for the extra speed, the
unique exposure modes it has, and wireless flash capability using the built
in flash.  It can use the newer lenses like the 16-50 and 50-130's quiet
ultra sonic AF motors which the K100D can't (the K100D Super can).

I've run out of time now, hope this helps.

Steve B's fairly comprehensive reply sounds pretty well balanced to me.
The K10's a nice camera (especially at current prices!), but it's a
bigger lump to cart around, and that probably takes some getting used to.
However I'm considering one to update my 100D purely because my main use
for the DSLR (instead of my much more convenient and very capable
superzoom in most circumstances) is at airshows, and I spend a lot of
time cropping shots. So more pixels will definitely help me.

So, sadly, would faster focus speed. I'm planning a new camera + Sigma
50-500 before June, and at the moment I'll probably be tempted towards
Canon for the slightly faster, focus. In general use that's probably not
important (unless you have fast-moving children to aim at), but I found
my 100D's performance disappointing when aiming at fast moving objects in
the sky... no matter how well lit they are.

Andrew McP

PS One thing I will say is that having had a pretty capable superzoom,
and a DSLR for nearly a year, I think it's incredible what manufacturers
are able to produce from cameras with tiny sensors. The only time I reach
for my DSLR normally is when I know speed or low light or focus speed is
going to be the issue. The 100D may not always focus quickly enough for
my needs, but it's still a DSLR!

PPS I think any discussion of recent Pentax DSLRs has to mention noise.
When surrounded by Canons & Nikons at airshows last year I was surprised
at how much noisier my 100D was. It's not a subtle camera! This shouldn't
be important (especially at airshows :-), but if -- like me -- you
usually like to blend into the background when taking pictures, it's not
ideal. The K10 is quieter (sample sounds are available in the dpreview
reviews somewhere) but Pentax still need to work on this IMO.

The K10D is a good camera, pretty complex, a big control set and is
able to produce good images if you put some work into them. But also
consider Olympus's E-510 for "out of the box" quality, both images and
kit lenses.  Plus, it's a lot lighter, the K10D verges on the weigh of
some of the semi-pro models like Nikon's D200.