I started using the Nikon D5100 a couple of months ago and it has some fun features. My normal camera I use is the Nikon D90 and I am extremely happy with it. The D5100 has a 16.2 MP DX-format CMOS sensor and 1080p video and the D90 has a 12.3-megapixel DX-format CMOS imaging sensor and 720p video.
The D90 has two control wheels for control of the aperture and shutter speed. The D5100 also has the ability to adjust the aperture and shutter speed but only has one control wheel that you have to hold a button down to switch between aperture and shutter speed.
The D5100 has effect controls that allow you to create color sketches out of your photos or select up to 3 colors for the camera to process and the rest of the picture is black and white.
The D5100 is a great camera especially for beginners who want to get into the DSLR world and the 16.2 MP CMOS is great but I still prefer the ease and speed of the controls on the D90 for getting that quick shot.