Just saw an article at AmmoLand entitled ". 40 S&W : Is It Still A Good Ammunition Round?" The article is not as bad as the title suggests, but it also does not address the point which is: nobody has said that the .40 S&W is a poor defensive round. No one (at least that I've seen) is even saying that it doesn't deliver (slightly) better ballistics than the 9 mm. Rather, the issue is--especially with modern hollow-point bullets--that the difference in performance between 9 mm and .40 S&W is so negligible that the advantages of .40 S&W don't outweigh its disadvantages, which are more recoil, generally a slightly larger and/or heavier weapon, more expensive ammunition, and, generally, lower magazine capacity.
If you have a .40 S&W and like it, then keep it. If you want a .40 S&W--for whatever reason, whether that they are currently very cheap on the used market, you feel that whatever caliber you use should start with a "4" or you want the (slight) edge on penetration that it provides--then get it. But if you are looking to purchase a handgun and are on the fence between .40 S&W or 9 mm, my recommendation is to go for the lower recoil and lower ammunition cost of the 9 mm.