I don't get it. Why do people think their time on the island didn't happen? Since when did theories of there being an afterlife get thrown into genre of 'sappy'? Granted their portrayal of the afterlife is of Damon and Carlton's imagination - but hasn't the whole show been based on that premise?
From the beginning Darlton has said that the show is primarily about the characters. The island was and has always been more of a background for the story than the story in and of itself.
The show was ultimately about redemption. Think of the episode, "Dr. Linus" on steroids. The side-ways was about how they each learned to let-go. Therefore Jack dealt with his daddy issues by being a dad and seeing how his son viewed him the same way he had (maybe mistakenly) viewed his own father. Sawyer dealt with being a con-man by being on the other side of the law. Kate may or may not have been innocent, but she found her life's purpose in Aaron as did Claire. Hurley still won the lottery, but now he views himself as lucky .... etc ....
The moments we saw were the moments they realized what their real lives had been. The sideways was created for them to find each other. To connect to that part of their lives that had been the most important. To get them to let go. To move on.
The island all really happened. I find it so odd that that was stated over and over again and yet still some people missed that. Whatever happened really happened. Jack really saved the island. Hurley really took over. Ben really stayed behind and helped Hugo. Lapidus and Kate and and Claire and Sawyer and Miles all really left the island. Who knows how long they all lived after that. Kate states that she missed Jack. Sawyer tells Juliet that he "had her" referring to the last moment he saw before she died.
I really thought this was an example of story-telling at its finest. I'm still so honestly baffled by people's mixed reviews!