cliff notes:
has a long history, goes tru phases of being called a wonder supplement and garbage on the boards lol.
the most recent studies, which suppossedly were also the best designed and contyrolled have shown decreased catabolism when dieting, and increased muscle mass when sufficient calories consumed.
its included as part of a supp called "Juven" by EAS that is actually reccomended by docs and AIDS groups to AIDS patients to help improve strength and fight wasting (its hmb, arginine, and glutamine if I recall)
for me the best evidence is that its a refined metabolite of l-Leucine, which has ALOT of new studies demonstrating positive protein synthesis benefits, especially in over 30's (but for everyone). Leucine is the basis of the supp Leukic by muscletech. if you read their ad claims, some of that is lifted directly from study results. very impressive.
I use it, and I'm a skeptic