I have been a fan of lost since the beginning and I saw final episode this week. Lost has been there for so long, you know all the characters and they become like friends you look forward to running into each week. They won't be there any more.
At the same time, there is probably no way for the writers to successfully wrap up such a complex mess of mysteries and layers, backstabbing and lofty, noble enigmatic goals. I can vouch for that - they just didn't succeed. I'm sure there is a writer sitting back saying, "There. All done. All the mysteries explained.", but he would be wrong. They pretty much did not explain anything.
First of all - what the hell was the island? If you can't answer that, you've failed miserably. Does it exist in the real world? Can everyday, ordinary people actually get there? What was the smoke monster? What is the light at the centre of the island? Who was Jacob's mother? Where did she come from?
Too many loose threads for a successful ending. There is a bit of closure, in that you know where everyone is at the end and they are all happy and there's a warm, fuzzy feeling, but seriously, if you think that even a single mystery of the last six seasons was answered, then you are delusional.
Overall - fail. I'm sure the studio made a ton of cash on it though.