I'm only a couple of years late, but I feel compelled to comment. I just finished watching Lost and I have to say that I was blown away.
I know, I know it's old news and opinions are fairly polarized from what I can tell, but I felt the ending was satisfying. I thought that the show had done an excellent job throughout of going back and showing you answers to questions, big and small. Even when we knew where the storyline would end up, there were times when I was baffled up until the reveal as to how it ended up that way. To me, that established a framework wherein just about anything, given infinite time (and which, of course, no one has), could be explained within the context of the show. Yes, there were red herrings and dead ends, but overall, I was impressed, and most particularly impressed with the intricacy of planning required, in the first three seasons anyway, to show events from multiple perspectives, answering questions or filling in gaps from season 1 as late as season 3.
I didn't know anything about the flash forwards until Kate stepped out of the car in the season 3 finale. Besides losing Charlie in that episode, the depiction of Jack as a broken man was, for want of a better word, haunting (I feel silly phrasing it that way, but it fits). For every new direction the series took, the show continued to maintain my interest, and it continued to deliver solid story-telling. The build-up over the flash deadward scenes towards the final episode, I thought, was amazing. Did the ending leave unanswered questions? Yes. Did it make empirical sense? No. Did I want to see more in the way of the much-talked about, but to my mind less evident, "mythology"? Yes. Do I think that any of this ruins the series or makes it a waste of my time? Absolutely not.
Anyway, sorry for chiming in late on a cold subject.