No side effects. Your body will piss out what you are not using, or find another function for it. I guess in theory it can go through a process and be stored in adipose tissue, but you will be needing to take a HUGE amount, and I mean HUGE for this to happen.
A defninite- Less Doms. I've forgotten to use my xtend a few times and always paid for it the next day by being extra sore than normal.
Also I feel I can have more prolonged workouts when I use xtend. I usually lift for an hour and forty minutes and do cardio for 20 minutes after. Without BCAAs I have to trudge through the workout but with it, I feel more energized and more willing to complete it with intensity.