1) I have quite a few customers who are professional photographers and they all use/recommendeither Canon or Hasselblad. I couldn't afford the latter[]. My experience with Nikon is pretty limited though: I have a D40 at work and played with my friend's D200 on several occasions.
2)Both Canon and Nikon systems are easy to use, one can adapt easily.
3)For the lenses I've tested, I amby no meansa pixel peeper, butthey certainlyget the job done.
4)There is one thing that sometimes drives me crazy: when a newly taken photograph shows up on your LCD screen, the only thing you can do is to look at it. You have to press play in order to magnify or compare with previously taken pictures.
In conclusion, each system has its pros and cons. I personaly like diversity. It would suck if allmanufacturers were making identical models of cameras..