I bought a Nikon D800, and have taken about 1000 shots with it over the last month. Its a great camera, however, it really wants the top Nikon Glass to perform, and even then, you need to be pretty careful. For sports and wildlife, Nikon has some great super teles, but nothing like the 100-400mml or 400mm f/5.6L, they are missing the boat for full frame users who are looking for something under $3000 or even under $2,000. Many of their normal under $3,000 telephoto lenses perform poorly with a TC relative to Canon.
To me, The glass is the most important part of the system, and, while Nikon used to have some of the best, they have not produced equivalents for some of the Canon lenses I use. No 135mmL equivalent, their macro does not match the 100mm L, nothing of the quality of the 24-105mm L, their new 35mm f/1.4 is way overpriced for a average quality lens, and, of course no 400mm high quality low cost glass.
Canon has produced some great lenses and so has Nikon, but I prefer the Canon glass.
I thought I could get by with Nikons three excellent f/2.8 zooms, but in low light wide open, CA's are the worst I've ever seen. I thought my expensive 24-70mm G was bad until I went to Photozone and found that they saw the same thing. Stopped down, they are fine, but I shoot low light with my lenses wide open, often at ISO 3200, 6400, and even 12800. I can use most Canon primes wide open and get tack sharp images, while with Nikon, only the 85mm f/1.4 is really special.
All my Nikon stuff I bought over the past 2 months is for sale, and I'll stick with Canon.
However, each brand has its strengths and weaknesses, go for the one that fits your style.
