Right now, the internet is abuzz with various reviews on the series finale of Lost - so many, there is really no point trying to add yet another one. The interesting thing is, that all reviewers essentially analyze episode correctly, but their final judgment differs, mostly due to the reviewers' expectations and personality.
In a nutshell, the last episode of Lost is more than reminiscent of the last episode of BSG (my thoughts on that can be found here) - it focuses much more on the characters, sacrificing on the actual answers some fans had hoped for. Some are going to love it, some are going to hate it. But with a show like Lost, it would be impossible to please everyone.
In the end though, Lost will stand the test of time. No other show (and I honestly mean that) has managed so successfully to combine a mainstream sensibility while demanding constant attention from the audience due to its complex writing. And it has managed to keep its quality to a constant level throughout its span.