I usually use Levels for slight "exposure" changes on JPGs/TIFs, when the RAW isn't available or when the adjustment is so minor that it's not worth going back into the RAW to correct.
I use Curves for contrast adjustments. I think the control you have in curves over the whole general image is much more intuitive and allows you to utilize standard "rules", whereas Levels seems to be, for me, a much more case-by-case basis kind of deal.
I guess, in a sense, I use Levels for "Brightness" and Curves for "Contrast", since I never end up using Brightness/Contrast for anything (they basically remove information, from my understanding).
I end up using levels layers for all of my blending layers, though. I find that the adjustments I can make in levels for these layers ends up giving me more flexibility and more of what I want in my end result, whereas I could probably achieve the same thing with curves... it would just take me longer.