I like the 4/3 and micro 4/3 cameras better than the full blown SLRs. Not having the mirror makes the camera bodies lighter and more compact. In the past, a real optical view through the lens via the mirror was better, but the digital viewfinders on the new 4/3 cameras are so good now, I think they're better than the SLR view - they work better when it's too dark to see properly; there are optional functions to zoom the viewfinder or provide 'head up' displays. Also it's easier to fit all your existing lenses from older cameras (even film SLRs!) - you don't get autofocus with the old lenses but it's nice to be able to use them: the focal lengths are effectively doubled so become more telephoto when used on the 4/3 cameras.
However, if you're a Nikon fan, I don't think Nikon make a micro 4/3.
