After looking at the prices some companies are charging for stripped lowers and parts kits, I'm not totally convinced a DIY is a money-saving route. Also, I haven't really seen two not-same-brand receivers (upper and lower) with a real positive fit, necessitating use of a wedge to stiffen them up.
Having said that, I have built several ARs for personal consumption, but did so from "80%" lowers... meaning I had to machine them myself. This was the only way I could do a DIY with a good fit between the upper and lower, and that was a halfway reliable home-built. If I didn't have a mill and lathe in the house, I would definitely go store-bought. (Heck, I have the mill and lathe, and still go store bought half the time!)