So the IQ of Nikon lenses ends up being this enigma with no definition outside the usual comments.
There are lots of tests of many lenses - photozone, SLRgear, DPReview, etc. all test both Canon and Nikon lenses. There's no mystery-wrapped enigma, in general (although there are obviously exceptions) where both have a similar lens, the IQ of the Canon lens is better.
Now, if you specifically mean the supertele lenses, there aren't many tests of those from either brand. What tests there are favor Canon, which given their advantage at shorter focal lengths, use of fluorite elements, etc., makes logical sense.
Who knows - maybe the Nikon camp is running the tests, but are too embarrassed to publish the data... (kidding)
The enigma comment was concerning the supertele lenses. I've been looking for comparisons of the 300f2.8 specifically since it's a good chunk cheaper than the Canon. Reviews are slim and cross brand comparisons almost non-existent.
Unless you start adding TC's to one, they are both pretty good. I don't have a Canon version, but here is a shot with a older Nikon AF 300mm f/2.8 mounted to my Canon 40D, so it was manual focus and at f/2.8, the depth of field was very shallow.
Here it is with a D40X I had at the time. It appears to have been front focusing. Nikon had those issues too.