After discussing the show with fans on Lostpedia for the last 6 months who either loved or hated it, I've come to believe your experience with the show is personal and depends a lot on your expectations and perspective.
There are too many people who absolutely LOVED it and too many people who are devastated by the ending for there not to be some extremely subjective reality to the quality of the story.
My personal experience was that I loved seasons 1 + 2, got disillusioned by 3 + 4 because I thought they didn't have a way to wrap any of it up, I was really just watching it because my husband wanted to stick with it.
But then towards the end of 5, culminating in the season finale "The Incident", they answered enough of the questions about the mythology that I was completely won over and season 6 had me back to where I was in the first two...
However, I'm someone who not only likes sci-fi, I really love mythology and spirituality, metaphor and the like. I can let go of the necessity for scientific explanations... I didn't have huge expectations that everything was going to be answered, perfectly, in fact, just the opposite. I didn't really like the characters much by the end of 5, but season 6 wrapped up their stories and explained why the fact that they were so flawed was important. I started to care about them and found myself routing for even the ones I didn't like the most... and the way they resolved most of them I found extremely satisfying.
I would never say LOST was perfect, or that I wasn't disappointed that some questions were left unanswered or were there for the sake of mystery - and I totally get why people hated it and felt cheated. I just wasn't.
Also, I understand the writing process was limiting.
It seems to me that Lost was written like the first draft of a novel. They had a great premise, great characters and a sketchy storyline with a general idea of the ending and they just went for it without a detailed outline that planned everything out.
And then the writers strike, and actors bailed and other actors (Micheal Emerson), showed up as brilliant additions - and they had to or wanted to change things along the way... Only, the 'chapters' were published episodes that everyone had seen - they couldn't go back and edit out the pieces that they didn't have time to resolve because of the deadline.
So they did the best they could. I would have loved another season to wrap it up, but it is what it is....
BTW, I got so hooked on Lost and had so many issues trying to let go of unanswered questions that I wrote
4 fan fictions to answer the questions and help me cope. :lol