I am a Nikon shooter. The D5100 is a very good camera. It has about the best DX sensor in it that one can get. It is the same sensor as the D7000. It has great high ISO capabilities. The only problem I see with it verse the D7000 is that it will only take the AFs lenses(Built in focus motor). This can be a big deal for Nikon shooters because if you have a body, like the D7000, you can use older Nikon glass that dont have built in AF motors. If you are thinking about getting serious, even a little, I would go with the D7000 if you can swing it. The D7000 will also meter with the old Ais Nikon glass. D5100 is a very capable body though and there is nothing wrong with having AFs only lenses.
Not sure if this all makes sense. Just do your research and know what you want to be able to do with the camera. Most on here shoot Canon, but there are a few of us Nikon shooter.