I just switched from Canon to Pentax. I owned a 300D & 20D and used the 30D and the 40D.. in fact it was the 40D that made my final decision to split! Canon was not listening to their customers and was not giving us anything wanted nor needed. They were adding a few new features and calling it a #0D as a new camera!! They never bothered to fix any of the issues with the 20D nor the 30D, even those that most likely could have been done with a Firmware Upgrade! They don't bother to recall over major problems, but instead sweep it under the carpet as long as they can first!
Aside from the above.. The K10D offer weather sealed body, better feel, beet image quality, better DR, far better RAW capabilities, better metering, true spot emtering (not that 3.5% BS Canon calls Spot), etc. Oh yeah, Pentax has a cheaper price too!
The K10D is nothign close to the Rebel/XTi, that would be the K110D or K100D. The K10D, I would actually rate above the 20/30/40D cameras, more towards the Series 1 bodies, but obviously not that good, lol.
The K10D without a batter grip felt better then my 20D + Battery Grip if that tells you anything, and the 20D, 30D, & 40D are all the same basic camera. Even the 5D is basically a 20D body with a FF sensor.
I sold off all my lenses, and I had a real nice line up too, to switch over to the Pentax K10D. I did already have quite a bit of older MF glass from all our film SLRs. But the only thing I regret about switching... is the fact that I didn't do it sooner!!!
Also, Canon tends to not produce very accurate colors!!! Whereas the K10D produces perfectly accurate colors! Canon's metering is also nowhere near as nice or accurate as the K10D either, atleast not on the 20/30/40D cameras.