I have a D3, A D2X and and a D300, but still prefer to use my D5100. As Ray has said, it's very comfortable upto ISO6400, and is very nearly a match for my D3 (just not quite).
Very light, very easy to use, good battery life, great Video (albeit without manual control) and excellent exposure, and excellent resolution from it's 16mp (D7000) sensor.
Actually, because I was so used to my D3 and D2x bodies, it felt too small, so I added a really cheap Mielke grip to mine, and it improves the feel 100% (and takes 2 batteries).
The articulated screen is a joy to use, and certainly makes it more expensive bigger brothers look a little backward on that front. Absolutely love playing around with the effects modes (especially selective colour and minature).
The only thing I have changed is to select the custom button function below the flash to ISO, and have ISO displayed in the viewfinder rather than frames remaining.
Negatives, well it doesn't have a built in focus motor, so AFS lenses only (unless you don't mind manual focus), doesn't have wireless CLS master function for the flash, and only one command dial (not two like I'm used to). But to be honest for the price, I think it's a fantastic fun little camera, and coupled with decent glass would be hard to tell apart from much more expensive bodies.