Frank Field , Oct 18, 2010; 07:30 p.m.
I have been using a pair of D200 bodies for about 3.5 years now and do primarily landscape photography. Much of my work is at focal lengths of 35 to 200 mm. I have never been a huge fan of very wide angles for landscape photography so I do not feel pinched by using a DX-sized sensor and, in fact, prefer the added "reach" provided by the cropped frame sensor. As a landscape photographer, much work is done in the damper early and late hours of the day so I appreciate the moisture sealing in the D200. With depth of field a key priority for landscape work, one really does not want to go beyond a 10-12 MPixel DX-format sensor. Diffraction limits start to set in beyond f/11, I find f/13 to be a practical working limit and find that losses due to diffraction at f/16 tend to cancel out any advantage from added depth of field. Situation only gets worse if one opts for a camera with more pixels on the same size sensor (e.g., D7000). Most of my shooting is done at ISO 100 or 200 from a tripod; I will occasionally shoot hand-held at ISO 400. Noise performance of the D200 is OK at these ISOs while it starts to become quite noticeable at ISO 800. The D200 features mirror lock-up; I can't imagine doing serious landscape work, especially with longer lenses without this feature. Lastly, the D200 has the older style (~250 kPixel) monitor while newer bodies are more like 800 kPixel. Not enough resolution to judge critically focus accuracy tho fine for overall composition and display of three channel histogram data.
I have only thought of two features missing from the D200 that I would like to have: live view (for use as a focusing aid when doing macro work) and the ability to fine-tune autofocus. (The D700 has both of these features.) As such, I have not been tempted to upgrade to the D300 tho that would be my current DX-body of choice if I were to make a first purchase today.
Both of my D200 bodies have been trouble-free - one was purchased new and the other used. One now has 30k - 40k shutter clicks.
I don't have personal experience with the D90 tho it clearly has been a popular advanced amateur body. Data at dxomark.com suggest that it is the best performing DX-format sensor in Nikon's line (best in terms of noise performance, slightly beating the D300). I would suggest you check closely for features like mirror lock-up.
I have found a working 'kit' of one D200 with a 17-55mm f/2.8 and a second D200 with an 80-200mm f/2.8 meets most of my needs, keeps the 'kit' fairly simple and maximizes interchangeability of accessories. The lenses spend about 90% of their time on one body and I've had much less challenges with dust on the sensor since I minimized lens changes in the field.
All the above comments are from someone who primarily does landscape work. I know that would have different criteria and evaluation if I did distinctly different types of photography (e.g., weddings, etc.).