My husband is the king of addons. If someone, somewhere, devised a method of making his WoW experience just a smidge easier, he has probably downloaded it and tried it out. I, on the other hand, have this strange sense that addons are "cheating" in some way, and I don't use them unless I can't get through the game (or at least raid) without them! I'm not sure where I picked up that particular attitude, but one other significant difference between me and my husband is that he loves gadgets, and I'm not such a fan. Isn't that kind of what an addon is? A gadget to make things in the game more convenient?
That being said, I can't live without Deadly Boss Mods. All the warning noises and flashing words and warnings to Run away! make me look like I know what I'm doing, even when I don't. Of course, the raid leader requires us to have it, but I wouldn't want to try raiding it without it. Otherwise, I'd probably just hang out standing in the fire. For the record, my husband's favorite addon is Titan Panel. He likes that pretty much every bit of information about his characters is one mouse click away, but I feel like it's information overload.
What about you? What addon can you not live (or at least raid) without?