I over-medicated with 5-HTP - up to 300mg a day. I stopped dead because I was experiencing extreme, generalised paranoia at work, short temper, and everything just felt a bit "shit".
It could have been an atypical response, but more likely the excess 5-HTP interfered with my dopamine levels - there's a rate-limiting enzyme step in the creation of serotonin and dopamine which relies on the same enzyme, so an excess of 5-HTP could conceivably have dropped my dopamine levels horribly low.
All that said, I do find it helps with sleep at recommended (~50mg/day, ideally an hour before bed) doses, but your mileage my vary =o)