I took tribulus and nitric oxide while on a weights program and found a real improvement in the hardness of erection, but no size gains (I wasn't doing any Penis Enlargement at the time). It was rock hard and also prolonged the time it took to ejaculate.
I've tried tribulus in the past and I found it did work but only while taking the supplement. As soon as I stopped taking it, all of the positive effects went away. Quite expensive too.