I bought one and use it while doing conditioning work. I like it for plyo or H.I.T.T training drills, and for footwork drills and jump roping. Tried to run in it a couple times and immediately my knees started aching, so i put a stop to that.
My opinion, if you got the extra $, it can be a novel training tool. But ya get the same basic results by just doing extra reps or going a couple extra minutes....... that is my take. It fatigues you faster, but doing a 4 minute round instead of a 3 min round does the same thing, as does doing 20 weighted push-ups with a vest, do 30 (or whatever gives you the same level of fatigue) without, same principle. In short, the only benefit i really have found is that it can cut down my workout time, but i am not sure that is necessarily a benefit?