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.
I actually just answered within the context of sharing within a family... I also have my MacBook air, the agency Mac pro I use everyday, an agency iPad (used for iOS dev and website testing), and my daughters old iPod touch all using the same Apple ID! I'm really surprised that Apple allows 10 devices to share one ID. It's saved me a lot of money, but I certainly spend my fair share of it in the app store (~$300 this year).
