I payed $4,995 for a Nikon D2x pro camera in May of 2005; the sensor in the D5100 is ***substabntially better****. Seriously--wayyyyyyy better sensor in the D5100. It can make terrific photos if you just point it the right direction, and click at the right time.
The capabilities of the D5100 are far higher than what early-gen pro d-slr cameras like the Nikon D1 or Canon 1D (original) had.
You also have the cool flippy screen, and can shoot video clips with the D5100. It CAN use old, cheap pawn-shop type lenses that retail for $29-$49. it will not autofocus these old lenses, but it WILL mount them, and shoot them!!!
The "baby Nikons" like the D3000 and D5000 series LACK the Ai Coupling system of higher-end Nikon bodies, which makes these "baby Nikons" also able to use Realllllllllly "obsolete" lenses like what KEH.com sells as "non-AI or Pre-AI" Nikon lenses....these 1959 to 1976-era lenses are often VERY inexpensive. The 55mm f/3.5 PRE-AI Micro~NIKKOR macro lens for example, can be had for $29 to $37 or so in BGN grade from a place like KEH. Tis, specific lens has a small frotn element located wayyyyyyyyy, deeeeep down inside the filter thread ring, so it is its own lens hood, and protector! THe outside barrels of a BGN 55/3.5 Pre-AI will look like rubbish: yet, the way this lens was built and designed, it will usually still focus silky-smooth! This is actually a "feature" of the D5000-class cameras: they can use OLD, low-cost, Pre-Ai or Non-AI Nikkor lenses, in full-manual-only mode of flash,and full-manual settings over f/stop and shutter speed, on the D40,D3000,D60,and D5000-class "baby Nikon" bodies!
There are millions of these pre-Ai lenses for sale-dirt cheap! Great for experimenting and for FUN! ALSO: 135mm f/3.5 or 135mm f/2.8 pre-AI era teles are often very sharp, very good lenses.