There is some good info in this article
I had to flip my stem over Christmas as I have pinched a nerve in my lower back and bending too far forward strains is not good for lower back posture. So my hands are a little higher but I am much more comfortable.
Paul, I rode in the drops most of the way as I have an integrated stem and bars that seems to fit me exactly now....use to get bad wrist strain but when I went for the new bars I went 5mm longer in the stem....no more wrist pain :)
The geometry on my last bike and new bike are like chalk and cheese. The old bike had a very aggressive geometry and I found with the seat post at the correct height I couldn't get comfortable on the hoods let alone the drops! This made me think the frame size was too small for me but the new frame is the same size... The new bike is perfect for me in comparison, reaching the drops is comfortable and I'm using them more and more (including during the downhill parts of the Bupa ride). I certainly notice the aerodynamic difference on the drops but I tire fairly quickly. I think as DB mentioned, you use different muscle groups on the drops but they'll strengthen with more frequent abuse.