I tried it several years ago. I took it for about a year. I read a recommendation somewhere that you should cycle it, so I would take it for five days in a row and then skip three days.
I did seem to notice some positive effect, slightly increased energy level and slightly increased libido, etc. But that could have been a psychological placebo effect. Also, at the time I was doing a lot of weight lifting, which can boost testosterone by itself. So the effect might have been from that instead of the tribulus.
After about a year, taking tribulus seemed to start boosting my blood pressure and heart rate a bit, so I stopped taking it.
Tribulus appears to me to be another one of the many things that can have a certain amount of limited benefit for some men. But I don't think that it's anything to get too excited about. The manufacturer of one brand even said that it's benefits disappear after you've taken it for about six months, because your body gets used to it or whatever. | 
