Bijou questionette: SAD lamps

I'm thinking of getting myself a SAD lamp for Christmas (well, technically I'll be getting it on behalf of my mother, who'll be getting it for me, but that simplifies nicely to it never actually passing through her hands).

I'm not sure that I technically need one currently on medical grounds (there has been some occasional historical wonkiness in my vitamin D levels), but I tend to see very little direct sunshine in the winter given my body and cold have a longrunning disagreement, so a SAD lamp might not be a bad idea.

Anyway, I'm fairly certain I've heard one or two people around here mention using one, so simple question: Are there any features I should particularly be looking for? Or avoiding, for that matter? 

Any and all input gratefully received.