When Winston succumbs to the torture and accepts his place in society and love for Big Brother, does that change of heart seem sincere?
This answer is definitely going to change based on individual readers. Yes, I think Winston's love of Big Brother is sincere. I don't believe that Winston would have been let go unless his feelings were sincere. In part 3, chapter 4, O'Brien asks Winston how he feels about Big Brother. O'Brien says that he will know if Winston is lying, so Winston tells the truth.
"Tell me, Winston -- and remember, no lies: you know that I am always able to detect a lie -- tell me, what are your true feelings towards Big Brother?"
"I hate him."
"You hate him. Good. Then the time has come for you to take the last step. You must love Big Brother. It is not enough to obey him: you must love him."
He released Winston with a little push towards the guards.
"Room 101," he said.
When the book ends, Winston is free of his captivity, so I would presume that O'Brien successfully got Winston to say that he loves Big Brother. O'Brien would know if Winston is lying, so having Winston simply say the words wouldn't work. In order to be let free, Winston had to have truthfully said that he loves Big Brother. The final paragraph of the book supports this idea.
But it was all right, everything was all right, the struggle was finished. He had won the victory over himself. He loved Big Brother.
Winston has been so brainwashed by O'Brien and the torture that he has embraced Big Brother and everything that he stands for. He isn't acting or putting on an act to stay safe. Winston wholeheartedly loves Big Brother more than anything else. That's probably why his interaction with Julia in the final chapter didn't stir any kind of emotion within him. She is simply a part of his past that doesn't matter anymore. All that matters is his love for Big Brother, and that is why the end is so disheartening for readers to read.