My, my, what a little love-in fest this is.
Careful folks, we wouldn't want this to become a cult. :-)
All cameras have their advantages and disadvantages.
Just a quick summary:
Pentax - access to thousands of compatible lenses, better 'bang for buck' then many, relatively great build quality (but darn it, each camera has one or more Achilles heals)
Canon - one of the most respected brands, superior to all others in low light/low noise photography, top quality lenses, good selection of professional accessories.
Nikon - the most respected brand, with few exceptions - usually the most rugged cameras but at a cost, superior Nikor lenses, best selection of professional accessories.
It's good to love your camera but be realistic and understand both its goodness and its many flaws.
I own the K20D but I also feel that the Canon 50D is superior in many ways; that's okay with me! I purchased the Pentax because I have several thousand dollars in accessories and lenses. The live preview on the K20D sucks, noise is much greater when compared to even the older Canon 40D, I dislike the fact that there is no AA battery grip, there is no GPS location to EXIF attachment (like both the Canon and Nikon have), and as someone else already pointed out... those doors on the Pentax are nice - EXCEPT they are made of cheap PLASTIC. I would suggest that rubber may last longer than plastic.