Originally posted by jeztastic Thanks everyone for the replies and opinions. My questions comprehensively answered! Next - which to go for - 15mm or 21mm (assuming I win the lottery)...
Both are good, but completely different. One is a true wide angle (though not what I'd consider an ultra wide at about 24mm) and one is a wide normal. I consider 35mm in the normal range and 28mm to be a border FL straddling wide and normal. Since the 21mm is about 32mm, it's neither normal nor wide...but I love the FL since it can be used for just about anything, and I'd say it's pretty much my "normal" lens, bearing in mind I was a person that considered 35mm "normal" on a 35mm camera.
I got my 15mm at the end of last summer, and didn't use it a lot because my K-7 was in the shop 2X. Since then I've used my EX1/TL500 quite a bit along with my 645N. So the APS-C DSLRs have taken a break and so have the lenses.
However, I have had the 21mm for several years and it's my all time favorite lens. I shoot it better than 50% of the time.
F3.2 is pretty close to f/2.8 (just 1/3 of a stop), and the K10D is perfectly capable of being used above ISO 400. Shoot in RAW, learn how the camera reacts to exposure, and learn how to process. The difference between it and the K20D was marginal, and the K-7 was about the same as the K20D. So the only camera you are currently missing out on is the $1500 K-5 which is obviously a significant improvement.
The K10D is a really good camera, with one of Pentax best image file outputs, don't fool yourself into believing you cannot create wonderful images with it. Yeah, it's getting a little long in the tooth, and feature wise it pales in comparison to the K-7/K-5, but the image quality doesn't!