My wife and I share the same Apple ID for our three iPads (don't ask), two iPhones, two iMacs and one Apple TV. We see each others iTunes music and iBooks. That can be annoying (my wife has a bejillion apps) but most of the time it's a benefit. Messaging on the iPhone is not shared, and we haven't used FaceTime, so can't comment on that. All in all, it works well. I wish we shared the same Amazon account, so we could read each others' Kindle books easily (we can still do it by signing out and signing in with the other's email, but it's a pain) but we started off on the wrong foot there.