I switched from Pentax to Canon. My K10D's AF got stuffed with my 50-135, so I did some "soul searching" while it was in Colorado. I always had some problems autofocusing in low light (especially with my FA 50mm f/1.4) and I was frustrated by a lack of a 70-200 (although the 50-135 was a nice lens)....So I said "what the heck?"
Sold off everything Pentax (most of it here) as well as a guitar and a high-end microphone preamplifier, and outfitted myself with a 40D and a bunch of nice lenses (including two big white Ls) and a flash.
What I like about Canon - FAST autofocus, even in really low light. Excellent high ISO quality (at least for a non-FF camera), lens availability (Pentax still has nothing new above 300mm, I have the very nice 100-400mm L now). ring-USM is awesome
What I liked about Pentax - Ergonomics (I prefer how Pentax does things, but I got used to the Canon), SR for all lenses (oh how I wish my EF 85mm f/1.8 was stabilized), the K10D/grip was a much better matchup than the 40D/grip, the lenses were optically excellent (I had the FA 35mm and 50mm and had used some of the Limiteds and FA* 85mm on occasion) Ability to use the pop-up flash to trigger external flashes (Nikon also does this, it's a mystery why Canon doesn't do this)
I found the 580EXII more or less on par with the 540FGZ, except the 580EXII has a better locking foot.
So in some areas, the grass is greener, in others, it's still filled with weeds