I did exactly this about a year ago when the 7100 came out and prices dropped on the 7000. The D5100 was my first DSLR after shooting an Olympus OM2n film camera many years ago. I liked the D5100, but wanted more flexibility in controls so I purchased a factory refurb D7000 from Adorama. I personally like the way it feels in the hand and the controls are so much better when it comes to making quick adjustments. It is bigger and heavier, but just feels right. As for any performance differences, I can't say as I am good enough to notice anything major in picture quality. I upgraded pretty quickly from the 5100 kit lens and that was very noticeable.
I really like having the dual memory cards, and the better flash control for slave flashes. Overall I am very happy with the upgrade, it was money well spent after selling the 5100 body. When I purchased my 5100 I couldn't justify the difference to get the D7000 for my first DSLR, but since prices have dropped especially with the refurb option I am very glad I upgraded later on.