I had a slow drain in the kitchen sink since I moved into my apartment. Recently, the bathroom sink started slowing too. I put this (one packet) in the kitchen sink, left for six hours and flushed with boiling water. It must've worked its way through the pipes because both drains were cleared within hours of flushing the kitchen drain. Oh, I did not notice any kind of funky smell and my apartment was closed up overnight while it was working.
Bottom Line: Yes, I would recommend this to a friend