I'm with a few of the others in this thread with respect to criticism. From my limited experience, it's not polite to volunteer anything that isn't both positive and true, and if asked, the worst you should generally say is "you sounded good tonight." Anything further should be first, invited, and second, presented in the most positive, constructive fashion possible.

Most criticism I've personally received has been on youtube, in which case I delete it (if offensive/personally insulting), ignore it (if useless or irrelevant), respond sarcastically to it (if it's extraordinarily stupid and/or presumptuous), or keep it in mind for next time (if it's useful).

Criticism I receive in person, I generally acknowledge or thank the offerer, then act on the information as I need to. If it's excessive and/or dumb, the "smile and nod" approach, followed by an immediate purge from memory, is best.

Just my opinion.