DOF preview is nice to have if you are doing careful work where focus needs to be finely controlled. That being said, it has its limitations:
- It is often hard to visualize due to a dark viewfinder (mentioned above)
- Even in the best case, what you see in the viewfinder is only an approximation of what you will see in the final product
Some people will say "huh?" to the second point. After all, isn't WYSIWYG one of the selling points for an SLR? Sorry to say, but what you see in the viewfinder is not what you get. DOF depends on both the virtual size of the viewfinder image, the physical size of the final image, and the viewing distance to that image.
In case you don't believe me, consider that the preview image on the camera LCD always looks sharper than the same image enlarged to 4x5" on your monitor. Ditto for thumbnails versions.
Moral of the story...don't pay extra for the feature unless there is a good chance that the camera's owner will develop an active and serious interest in photography. If you do have DOF preview, be aware that your mileage may vary.
Steve