I don't backseat game, mainly because I hate it myself. I do hate it when you have little kids come over and they think they no more than you, when they've been doing the hobby for about a day. Although, I do like it when an actual pro comes along and gives advice to both players, but not overdoing it by taking over literally.
I've never really experienced back seat gamers that much, I think the only time was when I was at GW Salisbury, and there was some complete idiot trying to get me to do this and that. I didn't want to considering I was doing what I wanted, not what he wanted.