I had one when I was breaking up with my Ex (speicifically during the period when she wouldn't move out!!!) Not something I would care to repeat.
I lost perspective on everything and starting behaving totally out of character. I varied between feeling apathetic towards literally all aspects of life, becoming very angry and violent at the slightest thing and then feeling unreserved and intense fear. Apparently they can take on many different forms though.
Ugh, nasty business.