I think Layne Norton did a lot of research in this topic. If I remember correctly, he thought that you can replace some protein throughout the day with just BCAA's.
I take them if I'm eating a meal/snack without a good protein source. Like a cheap grilled cheese sandwhich and soup. I picked up some Amino 2222 tabs from GNC on closeout. You should see the size of these pills. If you have ever taken a Kirkland multivitamin, they are about 3 times the size.