They do help slightly with recovery (taken after a ride). I've taken them before/while riding and haven't noticed a huge (any) difference (power/speedwise). I think they do allow me silghty more endurance than without them (but only to a certain point). But when going for speed, my legs still burn and when going for distance they still get tired. I haven't "scientificlly" tracked it.
As the others have said, there is no substitute for training.
L