I really liked this episode, but what disappointed me was the lack of resolution at the end of it.
John Noble did a fantastic job with his performance as Walter in this episode, and his shift from brain-damaged to brillaint scientist after the procedure was extremely well done.
I liked Nina's role in this episode as a sort of catalyst for the latter half of the episode, and the grey hair suited her.
The whole Orwellian vibe of this episode was probably my favourite part of the whole episode. Observers everywhere, curfews, Observer propaganda and even officers loyal to the Observers.
Also, the Brooklyn Bridge painted black was awesomely creepy
The revelation that William Bell was part of Walter's team was unexpected (since he 'died' at the end of season 2) and now I wonder what will happen with that hand that Walter stowed in that bag...
I too suspected that Etta was Peter and Olivia's daughter from the start (she looks
so much like Olivia), so I was happy that I was right when her family history was finally revealed.
I mentioned above that I wasn't happy with the lack of resolution in this episode and here's why: we're left with more questions than answers.
First of all: Why did the Observers choose to travel back to 2015 when the world fell apart in 2609? What was so special about that year that made them want to travel back, and since when did Observers become so violent? It seems strange to me that beings that are usually peaceful and non-interfering would suddenly change like this.
Secondly: Where is Olivia? It didn't appear that she was encased in amber with the others, so was she killed in 2026 (as we saw in "The Day We Died") or did she die earlier before the others were encased in amber? And is that why Etta has a bullet as a necklace; is it the one that killed her mother?
Thirdly: Did this happen in the other universe as well? I'm very curious as to how the other universe reacted to this sudden invasion, because if the bridge was still active then the Observers could cross unhindered. So was the bridge sealed off, or is the alternate universe under the same control that ours is in 2036?
Maybe that's where Olivia went!

(To the Other Side)
Lastly: What about that device that Walter was going to build? How exactly does it stop the Observers? I wish that they had expanded on that more.
Anyways, I rated this episode Very Good because the new approach by the writers was fantastic and I'm a sucker for anything dystopian. But for all its flair, this episode leaves us with more questions than answers and by the looks of next week's episode they are going to be unanswered for a little while (if not indefinitely).