I liked it quite a bit. The main reason I loved the finale was that emotionally it was such a rewarding episode, and a lot of those side-flashes eventually paid off in a very satisfying way emotionally. Whether the mystery on the island could have taken other turns, that's a whole other conversation, but I think the spiritual ideas of having the characters meet up again in the afterlife was fitting, considering the whole show (since s1) had been about the characters, how they were all connected and the fact that it wasn't coincidence that they all ended up on the island in the first place.
Personally I didn't mind where they went with the story on the island either. Some of it was really cool, and it wasn't what you would expect. My only real issue would be that when you look back at it after seeing S6, there's all these landmarks, places and let's not forget other people who show up on the island, and it just seemed a bit weird that they didn't stumble upon some of them sooner. Sure, a big island, they kept mainly to their safe places, but still!