This isn't a Pentax or K10D bashing thread - quite the contrary. I've always been amazed at what an incredible value the K10D represents, and perhaps that is why it's so frequently compared with cameras that are three to four times more expensive.
Nevertheless, for some time I've had three major complaints that have significantly limited my satisfaction with this camera: slow AF speed in low light, limited high ISO capability, and a 1/180 sync speed. I'm well aware that these may not be issues for many others, but since I shoot a lot of portraits both indoors (in low light) and outdoors (in bright light, where 1/250 would be very welcome) they are very significant for me.
So, after a lot of wondering I decided to rent a 5D to get a sense of what the competition has to offer. As has been pointed out many times, it's not fair to compare the K10D with a 5D that is four times more expensive - but that's what I'm going to do. Why? Simply because that IS exactly what I'm doing in my decision making process... trying to decide whether a 5D is worth the extra expense to get the features I want.
Enough with the background. Here are my brief (unscientific) observations.
* camera feels very solid and well-built
* AF is MUCH faster; frankly, I'm blown away at how much faster it focuses in low light than the K10D (note that I have mostly FA Limiteds, which are known to be slower than other lenses; I do have a Sigma 18-50 which is somewhat closer to what the 5D offers, but still significantly slower
* high ISO performance is unparalleled (except for the D3 & 1D); low noise without sacrificing detail
* a 50mm lens on a full-frame camera has a significantly different FOV than a 50mm equivalent (i.e. 35mm) on a 1.5x APS-C; it's been a long time since I used a film camera, so I had forgotten this; I prefer the full-frame FOV
What it comes down to is that if it weren't for my FA Limiteds I'd sell the Pentax gear and pick up a 5D in a heartbeat. They can be found for about $1900 new and $1700 barely used right now, so it's not completely out of my range. But the Limited primes are the reason I chose Pentax in the first place, and I don't know if I'm ready to give them up. I think I'll wait until late January to see what Pentax has up its sleeve before I make a decision. Also, there's a chance that a 5DmkII (or 7D, some say) might be released at PMA, and some on the Canon forums believe they could sell for as low as $2200 (low for a FF DSLR, that is).
Here's one at ISO 3200 (shot at 1600 and pushed a stop)