I've been using exclusively Canon digital SLRs for the past ten years and have a collection of L glass to use with my 5D. Recently I've been given some Nikon kit to play with by my college and I believe the D700 and D3s blow away anything that Canon has in the market place.
My story is this:
I had two EOS-20D bodies and a 50mm, 24-105 L and a 75-300. I never bought the EF-S lenses because I always anticipated the arrival of cheaper full frame cameras.
When I bought my first DSLR, the EOS-20D, I felt that the choice was pretty simple. The only serious contender was the Nikon D70. The 20D had slightly more megapixels but still considerably better image quality, especially at high ISOs. The autofocus on the 20D was also better. In general, the 20D felt like a much more solid camera than the Nikon D70. The only thing that bothered me back then was the fact that Nikon D70 had faster flash sync speed.
In 2008, I was going to upgrade to full frame. When the 5DMKII came, it was a huge disappointment to me. I had definitely expected to see a camera with less megapixels, faster frame rate, better high ISO performance and improved autofocus. Instead, they came out with a 21megapixel camera.; the high ISO pictures looked awful and overprocessed to me. The autofocus seemed to be essentially the same as in the 20D. Not necessarily bad but no improvement after 4 years of development seemed like stagnation to me, especially when I looked at what Nikon had in the D700.
After much agony, I decided to buy the Nikon D700 instead and start building a collection of Nikon glass . Yes, I am an amateur; I have never even tried to make money on my pictures. I do understand that for many pros, autofocus isn't that important if you are working with models that are posing for you and that megapixels may be more important if your customer demands 20+ megapixel images. However, I am an amateur and my situation is different and I prefer to use equipment that make sense to me.
I have been more than happy with the D700. It took some time getting used to the Nikon controls but now I am comfortable with them. I especially like the fact that the power button is together with the shutter button.
Add to that Canon's absence from the mirrorless and pellicle markets and I wonder what's going on, or not, at big red. Getting rid of a mirror on a trapeze flapping about is a significant and logical development but Canon seems stuck in the past.
I am a skeptic when it comes to mirrorless. At least I don't believe in EVFs any time soon. They would probably need to have much more resolution than what is possible today.
If we're forced to do that it will take massive technological innovation to persuade us to sell up again and come back to Canon.
Personally, I am still keeping my Canon lenses and flashes around, just in case they release a better camera than what's in the Nikon lineup. I occasionally even use my 20Ds, especially with the 75-300, since I don't have any telezoom for my Nikon. I wouldn't mind buying the 5DMK3 if it happens to be good enough.
However, I am still disappointed with Canon. I am especially disappointed with their aggressive pushing ahead in the megapixel race. I feel like their engineers are forced sacrifice image quality in order to satisfy the marketing department's demand for more megapixels. I think this is especially apparent when you look at cameras such as the 50D. Sadly enough, I suspect that Nikon is on the way to descend along the same path down to megapixel hell.