Leucine is HIGHLY effective at simulating protein synthesis and causing muscle growth. The research supporting this is overhwleming and beyond dispute. In fact, I consider leucine to be among the Top 3 basic nutritional supplements for bodybuilders.
However, there are 3 factors you have to take into consideration. Dose, frequency of use, and rapidity of digestion.
Regarding dosage, research shows that roughly 4.5 grams provides a maximum response in protein synthesis, with larger amounts having no additional effect (assuming it is delivered in bolus fashion and absorbed immediately; see rapidity of digestion below).
Furthermore, if you take leucine too frequently, the body desensitizes to its protein synthesis stimulating effect. So, the guys that drink a gallon of leucine/BCAA spiked water all day long are doing themselves a disservice. Leucine works best when taken at timed intervals of every 3-4 hours. This allows enough time for the body to become "re-sensitized" to its protein synthesis stimulating effect.
We also need to consider rapidity of digestion. Since administering leucine via intravenous injection is not practical (or desirable on a long-term basis), it works best when taken on an empty stomach, either 20 minutes before meals (my preference) or between meals. Taking it with a stomach full of food will drastically slow down its absorption rate, thereby diminishing its protein synthesis stimulating effect.
When researchers studied the 4.5 gram dose (the dose found to maximally stimulate protein synthesis) it was administered in bolus fashion on an empty stomach. Leucine needs to hit the bloodstream all at once (or close to it) in order to have a maximum impact on protein synthesis. Otherwise, the 4.5 gram dose will not work optimally.