It is the new ADHD. Normally not diagnosed until late teens and early adulthood, I see more and more kids with this dx. You're right in that it is difficult to diagnose, and there are several subtypes.
IMO, I think clinicians are sometimes confusing this disorder with acting out behavior caused by good old-fashioned poor parenting, poor schooling, and an overprotective, overindulgent society. Kids react and respond to their environment.
Garbage in, garbage out. I'm not saying every kid is misdiagnosed, but I believe many are, and it's a cop out. We need to take a look at what is really going on with these kids.