I agree the island side of the ending was good but the flash-sideways resolution was stupid.
But it is really stupid how vitriolic people get about TV shows. "Oh NOES, the hours of entertainment I got for free didn't end exactly the way I wanted to. The person who provided all that free entertainment is EVIL for basing his creative decisions on his own vision instead of granting all my personal wishes! I HAVE BEEN PERSONALLY SLIGHTED!"
The producer of Lost should go to the workplaces of all the people who are making those comments on Twitter and say "UGH, what an awful burger, you're a TERRIBLE fry cook!" Or, go to their parents' basements and say "My god you suck at Halo." Like the end of Jay and Silent Bob Strike Back.