Warlocks are OK as long as they get short rests all the time. If not, I would say they work very well in SOME settings. I played a warlock in a setting where magic was uncommon and outlawed and managed to play a "spider in the web" kind of character to managed to confuse and play people against each other by constantly shifting my appearance with Mask of many faces, and deception. He was ok in battle if he spammed EB, but the spell slot restriction is just ... wow.
