As a former D90 shooter, I can say without reservation that the
D5100
is now Nikon's biggest bang for the buck. The AF accuracy on this camera is much better than what I experienced with the
D90
. Image quality is astoundingly superior (i.e, great noise reduction while image stays sharp, sharp, sharp).
Contrary to what has been written by others, I find the "i" (Information menu) option for changing settings (e.g., ISO) to be quite fast and easy, once you "get" how to use it (in lieu of hunting for buttons or dials or button/dial combinations for everything). And, most importantly, the Command dial is still where it should be for quickly changing exposure settings.
In agreement with others, if you want a heavier APS-C camera and want to pay $400(US) more for all those buttons, be my guest and buy the Nikon
D7000
instead. Perhaps you need a larger viewfinder, or have older and/or AF-D lenses you want to keep, so you need a built-in AF (camera) motor to focus. If so, then spending the additional money for the D7000 might be worth it. But if you stay with Nikon's line-up, only a heavier/more expensive FX D700 can truly produce better IQ (and I would say perhaps, depending on conditions, because the resolution of the
D5100
is really phenomenal).