¡BIG LOST SPOILERS!

RE: Changing the past - I think it's the former, that they couldn't change the past, and season five was just a huge waste of time (for some of the characters). But just because they wasted their time doesn't mean we (the viewers) did. I think it's sort of unprecedented that they had this huge finale and this whole bomb/canging-the-past storyline and it all failed miserably. They were still in denial of the fact that they were actually brought to the island for a puspose, and in fighting against it they ended up killing one of their friends (Juliet). And then, once they realized it at the beginning of season six they all started dealing with it in their own ways: Jack finally started to 'give in' to the island, and James became a depressive loner.

But regardless of all this, on its own merits season five was great. The time travel storyline, the action, the mysteries. It was all so fast paced, entertaining, and well made. Remarkable television.