It's been less than 12 hours since the show ended, so these will be a bit half-baked. I'll keep it to three general thoughts:
1) So the flashsideways is technically taking place after the mainstream timeline, insofar as it's an epilogue set in some limbo- or purgatory-like afterlife that the characters are sharing. I didn't really see that one coming. I'm wondering whether there's a bit of a joke hidden in there, since 'everyone is dead' was one of the theories first offered to explain the mysteries of the island. But the tone of the epilogue would seem to suggest that this isn't the case. (So my complaint that I had no idea what the stakes were ends up a bit off-the-mark, since there really were no stakes.) And it provided the happy ending that would have only been bittersweet otherwise. We know that Kate leaves, Jack dies, and Hurley takes Jack's place - and we don't know what happens to them afterward - but we're told that it basically doesn't matter.
2) Shockingly, and aside from the reveal of the flashsideways-as-afterlife, there were no twists in the finale. Everything we learned about the nature of the island, about the Man in Black and Jacob, turned out to be absolutely true and we really didn't learn anything new about the island. It was straightforward, epic-drama - the world would end if Locke escaped and if Jack couldn't restore the light. And I thought it was pretty effective stuff, too.
3) So whether the show was 'good' or not hinges, I think, on whether you buy the overtly religious ending. That the show moved in this direction wasn't a surprised - the science/reason v. fate/faith opposition that so strongly characterized the first couple seasons was indisputably won by religion/faith side this season, and was probably a foregone conclusion as soon as Jack started to believe he was fated to return there. (Which is to say that, unlike in Battlestar Galactica, it didn't come out of left field.) But the strength with which the epilogue pushed a very Christian - that multi-faith stained glass window aside - resolution to the series was a bit startling. (At least they had the sense to make a joke about it, when Kate comments on the ridiculousness of Christian Shepherd's name.)
I found myself liking how heartwarming the epilogue was, but after this much time I'm starting to feel a bit cheated. I was moved in the moment, but I think that the credit for that goes to the actors and my own substantial investment in the show - I could ignore or miss the larger spiritual politics because I found it so damned pleasant. (This kind of sleight-of-hand is the same complaint I had of the Sun and Jin death scene, in a way - a touching resolution to their story that is at once undermined by the realization that they're abandoning their daughter, and probably abandoning her to Sun's father. Once the emotion of the moment wears off, you realize that they're actually pretty selfish.) I don't know whether there was a way to offer viewers that happy ending without going in the direction of religion, but, as-is, it feels incongruent with the first five seasons of the show.
But the ending on the island, with Jack dying in the field where he first landed, Hurley and Ben panicked about what happens next, and Kate leaving someone behind, again? That seemed more apropos, more like the Lost that offers half answers and an unexpected new status quo. Maybe the pressure to offer an unequivocal ending was too much. Maybe Cuse and Lindelof thought the flashsideways was more subtle than it turned out to be. Maybe the show was so hopeless at points that we needed the finale to remind us that hope and love exist and that the characters really care for one another And maybe the whole afterlife thing was the only way to pull that off. I feel some satisfaction from the characters' happy ending and I'm simultaneously numbed by the particularly Christian-styling of the message. I'm not sure what else I can add.