The ending is perfect
I just finished Lost for the third time. First time I watched was when it aired on TV, so I was 13 when it ended. I don't remember having much of an opinion on the finale, but I do remember other people saying how stupid it was. Second go around I was 17 or 18, and I absolutely fell in love with it, right from Episode 1 until the final episode, The End. It's been 10 years or so since I watched, and I only had fond memories of the show, but I only ever heard negative reviews of it from friends who had seen it. Because of this, I thought maybe I'd realize on this watch-through that it really wasn't as great as I remembered... But it was. Maybe better.
So many people who hate the show's finale feel that way because they think it was pointless -- they think that everyone on the flight died in the crash and that none of what happened in those six seasons ever mattered. They also think that the show went off the rails in seasons five and six. And I get it, the show did get a little stranger (time travel, flashes to what you're supposed to think is a parallel universe, a temple protected by a guy who seems like a samurai), but people seem to forget what they signed up for -- this is a show in which you accepted that there are polar bears in the jungle and a smoke monster that flies around an island that no one from the outside world can find. You accepted the supernatural/sci-fi/fantastical absurdity in seasons one, two, three, four... but all of a sudden the writers were "going crazy" as they tried to figure out how to end the show. But if you actually watch the show, you'll see that it was not a matter of figuring out how to end it... I think that the creative team knew exactly where they were headed.
I feel like I could go on forever here, but I'll just touch on the whole "they were all dead the whole time" theory. In the final episode when Christian is talking to Jack in the church, he explains everything (and, in my opinion, he does so quite clearly). He says, "There is no now... here" and that "Everyone dies some time, kiddo. Some of them before you, some... long after you." So, what he's saying is that everyone from the plane lived out their lives and when they eventually died, they were brought to this purgatory kind of after life and commenced their journey of finding one another and remembering. But since there's no "now" there, to each of them, it will feel like they just died, even though Boone died potentially decades before Hurley. Christian also says something along the lines of "everything that happened to you on that island was real." I don't know how people heard that and angrily yelled, "What a stupid show! They were all dead all along!"
Anyway, the whole show is perfect. To me, it is the most beautiful illustration of love, loss, forgiveness, faith (not necessarily religious faith), and the struggle between good and evil. I know most people I encounter who have watched the show will not understand the ending and will think that it was a waste, but to me, The End is perfect, just like the rest of Lost.