Having switched from the K20 to the 40D myself, I'll chime in on this one.
Coming from the Pentax, I will agree that at least in the beginning, the Canon button and menu layout seemed totally foreign to me and less intuitive than the Pentax. Now after a few shoots with it, I'm a lot more used to it now and can make it do pretty much anything I want it to do. I know some have complained about the LCD, but I really haven't found it to be an issue for me, even after adding a Giottos glass LCD screen protector onto it. Overall, the camera feels just as solid as my K20 did. For anybody who is contemplating trying one for the first time, having a manual or knowledgable fellow 40D user handy is definitely advisable.
Admittedly, I wasn't that excited about the colors that first came out of mine until I figured out that Canon takes a completely different approach to setting up the colors called Picture Styles; which one you would use depends on what you want to shoot--landscape and portrait are 2 examples. For each style, things like the colorspace, sharpness, saturation, contrast are set differently and are optimized for each subject. You can also create your own custom styles or download and add more styles to the camera itself. Since I use Lightroom for my editing, these styles will also show up in the Camera Calibration area of the develop module and will allow me to get the colors in LR that I actually saw on the LCD screen. Just in case you're curious to find out more:
Canon Picture Style
Just to give you an idea of what I've been doing with the 40D, I'll include a shot from each of the lenses that I currently have:
Tokina 12/24/4:
Canon 24-105/4 IS:
Canon 70-200/4 IS (can you say SHARP?!):
All in all, while I sometimes still miss my Pentax, I'm very pleased with the 40D and what I've been able to do with it so far.
Heather