While I cannot speak from personal experience regarding comparisons of Nikon glass and Canon L-glass, most reviews I have seen show the the Canon glass out-performing Nikon in key areas such as clarity, contrast, and overall optical rendering. Granted, the differences are minute and Nikkor lenses are extremely good, but that view is fairly consistent in comparisons I've seen.
I'm one of those people who wants to trade my Canon gear over for Nikon because of the better cameras Nikon makes, but that Canon L-glass keeps me from doing so. I'll have to see more reviews for better Nikon glass before I make that leap. Or, maybe Canon will save me the trouble and come out with a sensor that compares better with the ones in the Nikons.