I moved up to a D5100 from a Fuji Finepix non DSLR digicam.
I don't know if I'll ever move up to a full frame camera, but who knows, I'm toying with the idea of 'going pro' when I retire in a few years and see if I can turn hobby in to income.
I bought my D5100 in July and have been having a lot of fun learning the basics of DSLR ~ this website if great for help, even if they are a bit biased towards Nikon brand, but with good reason.
The one shortcoming of the D5100 that I was disappointed to learn was that it did not have the command system for Nikons creative lighting system (CLS) built into the camera. That is remedied by adding an SB700 flash or better and the flash will serve as master of the Nikon CLS. and you will be wanting to add an SB700 soon enough.
But to learn how to take control of your cameras settings I think the D5100 has more than enough controls for this photo enthusiast. And when in a pinch there is the "auto-everything" mode that is pretty fool proof to ensure you get some good pictures.