I've not been depressive in my adult life, but I have used 5-HTP before to boost my serotonin levels after they got depleted by certain substances, and it does the job for that. I also use it to potentiate certain things (make them more effective).
Taking just 5-HTP alone, for me, at 200mg dose, had some interesting effects. I felt less anxiety about things in general, just felt peaceful I suppose. I took it during a period of high anxiety (having deadlines and such, on top of having to spend a lot of time each day socializing), and it helped lessen the anxiety. The effect was best for me at 200mg or more.