I'm fairly sure this is spoiler-free, even if you haven't watched since season one.
Since the final episode of Lost started at 9pm on a Sunday night, I couldn't watch it live, though I wanted to. Wondering how it would end kept me from getting a good night's sleep on Sunday night; I kept dreaming scripts, usually involving it crossing over with other shows or other storylines, because these are dreams and they have to involve something surrealistic and absurd.
We watched it right after work yesterday. And I can say that I am entirely satisfied. No, they didn't answer every fiddly tiny detail question, but they answered all the questions that they needed to answer. Even some of the fiddly details that we despaired about ever getting answers to, I think they're there and we're still finding them. (I read a very interesting observation about the Hanso Foundation today that provided explanation for how a lot of seasons two through four fit into the larger story, for instance.)
Some things that are not explained are intentionally not explained because they are part of the premise: the story just happens to happen in a world where things like this are true. No one asks for an explanation for why Melinda can see ghosts, or why there even are ghosts, in Ghost Whisperer: it just happens to happen in a world where there are ghosts, and some people can see them. In the same way, some properties of the Island, and some other things like what Miles can do, are just how things work in this world, and while we do get explanations for what they are and how they work, we don't get explanations for why they are this way. Compare to how much better Star Wars was when the Force was the thing which binds the universe together, to when it was some nonsense about symbiont midichlorians. (And even that didn't explain anything, just pushed the question one level deeper.)
On top of having a satisfying resolution to the mysteries, the finale also had a satisfying resolution to the plot. The conflicts were drawn into sharp focus and then resolved. The challenges were built up to a climax and the tension ratcheted up, then the payoff made it all seem like it meant something. Even more, the characters were satisfyingly resolved, with each of the people we'd cared about given a chance to find completion, with some of the most tear-jerking, and entirely sincere, moments on TV in recent memory.
During the first couple of seasons, I considered Lost a really good show, well worth watching, though if it were cancelled I wouldn't've wept. Later, as they misstepped with the season about the Others, I got close to considering dropping it, and there were episodes I didn't pay strict attention to. Later seasons cranked things right back up, including the mystery and the complexity, but even more so, the characters. The show became more important to me again, but even coming into the final season, I didn't think of it as a favorite show. A few episodes this season were fantastic, but at no point did I even consider that this could be on my favorite shows of all time list at all.
Maybe it's just the emotional impact of the final episode being so fresh, but the way it all wrapped up just tied it so tightly in a bow, it feels like it retroactively made so much of what passed on the way here so much better that I am thinking it's probably on my top ten favorite shows of all time list. I haven't actually made such a list: I only know with certainty what numbers one and two are. If I took the time to make such a list it would probably be full of moments with me musing, oh, wait, that show belongs on it too, and maybe by time I was done Lost would have fallen off the top ten contenders list entirely. But even if that's so, it's remarkable that I'm even considering it now, when a week ago, it didn't even occur to me to ask the question.
Incidentally, the fact that I was at least half right, and more right than any other person I know of, in my theory about what was going on this season, probably isn't hurting either.