I have been diagnosed (surprisingly, too, as I was being investigated for something else) and while I know ADHD "exists", I will concede it's probably an overly broad compromise of a diagnosis for now. But then other well known conditions are too: IBS, fibromyalgia, CFS, for starters.

All these conditions represent something real going on, even if the currently used terms aren't precise. Much as celiac disease was considered a general bowel complaint by pre-1850s doctors before being eventually discovered, I think with ADHD we'll see more discrete conditions being discovered and understood too (e.g. a specific sub-condition that reduces the transport of dopamine in the brain, say). But, for now, ADHD is the term we've got to work with.