If you're working on a smaller, highly tactical idea then persistence is often a waste of time. Anyone who meets a lot of startups will see many ideas that just aren't going to work. The sooner the founders realize it and move on, the better off they are.
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If you're working on a big idea that is slightly ahead of its time, then persistence can pay big dividends -- check out the story of Pandora (http://www.businessinsider.com/pandora-story-2011-6).
If you're working on a smaller, highly tactical idea then persistence is often a waste of time. Anyone who meets a lot of startups will see many ideas that just aren't going to work. The sooner the founders realize it and move on, the better off they are.