I use both. 5d Mark 2 and D700. Both are excellent for what they do.
I thought I'd give my observations regarding both systems and little things about the Nikon system that I've been witnessing.
1. Bokeh. We're talking wide open or slightly stopped down. I find Nikon's bokeh is not as pleasing as Canon's. There's a buttery smooth look to Canon's, and Nikon tends to accentuate the round bokeh balls more, rather than just go into a buttery mush.
I think this has to do with Nikon's glass and how their build philosophy is different from Canon's.
2. I find that Nikon cameras cannot produce correct red colors. The channel gets blown when you try to push the saturation. Canon tends to have a deep red color with more definition, Nikon is more orangy. That's in RAW form.
Jpegs. If you are a jpeg shooter, there is no question you should be shooting Canon. Having owned the D7000 previously, and playing around with the D800, and owning a D700, I've come to the conclusion that Nikon just cannot do reds correctly.
Canon on the other hand can produce gorgeous reds with definition.
On the other hand, and I'll probably be flamed for it, Nikon's metering is SO MUCH smarter than Canon's it's not funny. Focus point metering on a semi pro body is awesome for getting correct exposure.
Also, Nikon's sharpness is out of this world. God their 50mm 1.8 G and 85mm 1.8 G lens is Much Much sharper than any Canon lens I've ever used. I wonder if this sharpness contributes to their harsher bokeh.