I would say though, that "surprise" visits are much less appropriate than planned ones. Your daughters are teenagers, so it's not as bad I wouldn't think as if they were toddlers, but my xw would often just pop in, and I wouldn't have to time to mentally prepare for it or more importantly prepare my daughter for it. My daughter would often get very blindsided, not when she came over, but when she then made an abrupt exit after she'd satisfied her need to see my daughter for a little bit. After all, you're not a coat to be worn whenever he feels like wearing it and then taking it off again, and especially neither are your children. But there is room for healthy co-existence with these visits, especially if you're getting along pretty well. They just should not be spur of the moment events, in my opinion.
M-34 XW-32 D-7 Found OM's presence 4/09 Separated 12/09 Divorced 8/10 GREAT relationship as coparents since 8/10