I'm going to put the rest of this behind a cut so I don't spoil anything in case you don't want to know how it ends.
OK so first... As an episode I thought it was great. Someone told me the original writers came back to do the end, and it makes me wish it had just been them all along. Remember before when that terrible Nicki & Paolo episode was an exception? Lately well written episodes have been so far between.... The Jin & Sun remembering scene was a million times more powerful than when they were reunited a few episodes ago, or even when they died together.
It was really nice to see the characters interacting again. That's something I have missed as well. It brought back that old feeling!
As for the end explanation...lol. Ridiculous! Purgatory? Come on :P OK I guess I can accept that as an explanation for this last season, but how does this have anything to do with the island? Dharma? The monster? The statue? Time travel? The only connection they made was that the things they did on the island together were "the most important events in their lives".
I guess wrapping all this up well would have been more than difficult, but that doesn't make the fail any less of a fail :P