I actually was excited about this episode and it disappointed--it turned out to not reveal a whole lot and instead looks to be a simple set-up episode for an ongoing LOSTesque type "flashforward" intercutting narrative format where we visit the present and work towards the future events in 2036.
Frankly I think with cancellation or at the very least a 13 episode final season they introduced this a little too late in the game.
The episode much like the series itself drew things out and padded the hour with a bunch of unneccessarily long scenes involving the mere mechanics of freeing Walter and restoring his mind leaving no time to really delve deeply into the Observers and the future--right now they are a vague generic plot device although I assume the writers are hinting cortexiphan and abilities they allow for is what leads to the Observers. We also have no better idea of the weapon to get rid of the Observers. And what few answers we got weren't very satisfying i.e. why the Observers took over the past. In short it was standard and univentive--in what has become the "go to" motive for sci-fi future threats most recently in The 4400 and Terra Nova--future humanity ruined planet Earth and sought to start anew in its past--yawn. And if the Observers can see the future and all possible outcomes you'd think they would see how the Fringe division would defeat them or how bad an idea it is to not wipe out all humanity since they will never stop being threatening and the Observer who got blown up in Massive Dynamic would have seen that coming
About the only worthwhile surprises offered up were William Bell was one of Walter's team and the switcheroo with Peter. The writers gave away pretty much from the start that Etta was Peter and Olivia's child so no surprise there.
Overall pretty average. I think we'll find out when all is said and done that much like Lost, Fringe will be remembered more for being good at throwing a lot of different plot elements into the mix, twists and turns that offered immediate gratification but ultimately very little in terms of providing satisfying payoffs to its myriad plotlines and a coherent bigger picture narratively.