Let's start with what prevented me from completely enjoying the episode. Look at the plot : someone creates a time-travel technology to be reunited with the woman he loved and lost, but that technology has to be shut down because it has lethal side-effects. Now, where have we already heard that ? That's right, "White tulip". I can't really blame the writers for trying to reproduce the recipe of one of their ( deservedly ) most loved episodes ... but this one was just too close.
BUT. There were really good scenes, anyway, as far as the relationships between Peter, Olivia and Walter were concerned. It was painful ( in a good way ) to watch how Walter ignored Peter, and the glimpse of Peter's state of mind ( and reminder of what he's lost ) with the opening scene was as heartbreaking as you'd expect. Just the look on Joshua Jackson's face when Peter realized Walter had become a recluse without him was outstanding.
Anyway, that's a 10 for character interaction, and a 5 for the case of the week ( even if the guest actors did their best ).