Nikon user here!
I went for my first Nikon, a D60, after trying out both that camera and a Canon 400D in a shop. I just preferred the general "feel" of the Nikon, it felt more solid and I preferred the control set up. I've pretty much stuck with the brand now on the basis of that decision, and to save buying a whole new set of lenses.
I've got a D300 now after feeling in need of an upgrade from the AF system on the D60 and it's ropey performance at high ISO settings, and it's quite honestly the best thing that's happened to me!
To be honest I think all of the big brands are more or less on an equal field, with some better in certain aspects but falling short in others. It's just a matter of personal preference and the type of photography that you do.