I did that for about 10 years from 2000 to 2010. The part about "being part of the furniture" is really on point. It is incredible how little they think of you. This has a side effect, they thrust you. Ebanking logins, emails, personal conversation, photos... You get it all. I was always very discrete and never did anything "bad", but there were moments when I was treated like a nuisance where I really wanted to. There is also a few times when you find out that the husband or wife is cheating, that is always very difficult to handle especially when the other one is here with you saying how great their partner is.