SPOILERS! Don't read further if you haven't watched.
I keep reading loads of people pissy because they believe that the sideways flashes meant that all the people on the island were dead. Even my mom texted me at midnight last night to ask if they'd been dead the entire time.
So, for clarification: What happened on the island REALLY HAPPENED. It was all real. Kate and Claire and Sawyer and Other Characters escaped the island and lived out their lives. Jack died. Hurley and Ben served the island for many, many years, and then they too died.
The sideways flashes showed us a shared afterlife created by the Losties after they'd died. It was an imperfect purgatory where they could experience life sans Island. It was a place where they would all be able to find each other and help each other cross to the other side (whatever that may be). It was a meditation on the nature of love and friendship and togetherness. True to Jack's word, it was necessary for them to live together so that they did not have to die alone.
But the whole show was not some long con. The events on the island really happened. The flash sideways showed us the importance of those events. The effect they had on the lives of the people who lived through them. They were so important, the people were so important, that none of them could move on until they'd reunited in the flash sideways.
So they weren't dead the whole time. If you still think they were, go rewatch the finale.
BTW, I loved that Ben, redeemed in real life didn't feel quite ready to move on. He still felt he needed more time to grow. It was wonderful to see that kind of clarity from a character who was so evil for so long. Redemption isn't easily earned. But it's possible. I think that's the lesson Lost wanted to teach.