As part of a project that I'm working on, I've just spent a lot of time in my digital image archives, which pretty much covers the full spectrum of Nikon mount DSLRs since the D1 appeared. A common question I get via email is "if I were to buy a used Nikon body, which one should I purchase?" Well, I think I have a pretty good handle on that answer today.
First, I'm going to remove from contention all DSLRs that can be bought new, so the D40, D60, D90, D300, D700, D3, and D3x all are not considered in my comments. That leaves the Nikon D1, D1h, D1x, D2h/D2hs, D2x/D2xs, D100, D200, D40x, D50, D70, D70s, D80, Fujifilm S1, S2, S3, S5, and the Kodak Pro 14n, SLR/n. Of those, I'd say there really are only six that truly make the grade and provide image quality good enough to consider against some of the current Nikon offerings: the D40x, D200, D2h/D2hs, D2x/D2xs, S5 Pro, and SLR/n. And of the ones that stood out, only two really distinguish themselves, but only when used optimally (which means keeping the ISO at or near base): the D2x/D2xs and the SLR/n.
And I mean stand out and distinguish themselves. It isn't just that the D2x/D2xs and SLR/n have more pixels than some of the others, like the classic D100 or D70. It's that small things like micro contrast make it obvious when one of those images come up on my screen. The Achilles heel of these cameras is different: the D2x/D2xs just stops being very usable above ISO 400, while the SLR/n is very lens sensitive. But at base ISO with the right lens, both produce images that'll hold their own against anything except a D3x.
At the other extreme, the D100 and D1x stand out as cameras that had clear issues that I can pick out quickly. That's not to say that I don't have any excellent images from those two cameras, but in direct comparison they just wouldn't be my first choice. The D100 had noise issues even at the base ISO (especially skies, meaning it was a channel noise issue), and produces soft JPEGs; the D1x tends to have very muddy shadow detail (micro contrast is fair overall, but poor in shadows).
So, if you're looking for bargains and don't need high ISO much, the D2x/D2xs would be my choice amongst used offerings. The D200 is probably the best "bargain" amongst the group due to its all around decent image quality, and you can find new in box samples of this camera for substantially less than you'd pay for a new D90; the D200 performs quite well overall. So much so that I had one converted to IR and it's my main IR camera now.
Another way of looking at things is "is it time to upgrade?" Here, I'll look at each body and make a judgment about whether it's time to move to a newer generation:
- D1, D1h, D1x: yes, to any of the FX bodies.
- D2h, D2hs: yes, to a D3 for more pixels, better really high ISO values; otherwise no.
- D2x, D2xs: no, unless you're moving to a D3x or need high ISO regularly.
- D100: yes, to a D300.
- D200: no; I'd wait for a D400.
- D40: if you need more pixels, yes, to a D60; otherwise no.
- D40x: no; present bodies don't provide enough more unless you move upwards in the lineup (e.g. D300).
- D50: yes, to a D60. (But note you may lose AF on some lenses, which may bring you back to no.)
- D70, D70s: yes, to a D90.
- D80: probably, to a D90.
- S1, S2: yes, to almost anything from a D90 up.
- S3, S5: yes, to a D700.
- Pro 14n, SLR/n: no, unless you need high ISO (D700) or lots more pixels (D3x).