I thought it was beautiful. I was surprised to discover how emotionally invested I'd become in the characters, even the ones I don't like. Also, it was the most compelling, most moving portrayal of Heaven (happy Limbo?) I've ever seen.
Once it became clear that many of the mysteries would never be solved, LOST took on an airy, dreamlike quality that I think enhanced the show. It became a show filled with characters that are intensely familiar in a story that's always a little disorienting, where you never quite know for sure where you are or where you're headed. Bizarre, magical things happen, but this isn't a police procedural where every mystery has a discreet, rational answer. The marvels are just part of the show's landscape.
My only real substantive critique: in my ideal ending, the show's ultimate hero would have been Ben Linus rather than Jack. His story just seems like a better allegory for the everyman trying to figure out What It All Means and led astray by limited, human explanations. That he ended up being a bystander in the climax, after all he'd done to shape the story arc since Season 2 or whatever, bothers me.