5-htp is the intermediate between tryptophan and serotonin. As far as I know, commercial 5-htp is not synthetic (good thing). Too much can cause nausea and sleeplessness. What is considered too much is individual, so start with low amounts if you try it.
I would expect it to work only for mild depression. Since it is an intermediary that relies on others (iron, I think) for conversion it might not be effective if other amino acids or nutrients or neurotransmitters are out of whack, too. Merely flooding your system with 5-HTP won't necessarily mean that any increased serotonin levels will actually bind to receptors and make you feel better as a result. That is the experimentation side of it, I guess.
This is talked about in the Autism community (my son is on the spectrum). It works for some and not for others. They recommend it be taken at night and with a Bcomplex vitamin and magnesium. 50-100 mgs/day is the amount they typically recommend for a child on the spectrum for anti-anxiety or sleep aid.
If you tackle depression, my guess is weight changes will follow. Additionally, better sleep can help with depression, too. I don't believe you should expect to lose weight because you are taking it, though. Shy away from anyone who will tell you this supp or that supp will make weight fall off.
Bottom line is: 5-HTP isn't dangerous so give it a (smart) try if you'd like. Good luck!