Alright, Cericme and I finished testing out Share Play, and there are definitely some mixed results. Towerfall off his PS4 worked perfectly, but we started having issues when we swapped over to my PS4 to play some Lego Marvel; Chariot; and Peggle 2. We kept having dropped connections, and Cericme was reporting some compression issues. Skylanders Swap Force actively blocks Share Play due to "content restrictions." Apparently the quality dips a bit on the guest's side when you're voice chatting, too.
That said, this feature is kind of a game-changer in terms of online play when it works. It was pretty cool playing multi off one copy of the game on games that didn't have online MP before.