Is it better to set unrealistic goals or achievable ones?

Overheard on Twitter: “Today I’m setting completely unrealistic goals. Really reaching for the stars, knowing that I won’t actually achieve them, but needing to free of obstacles like the word ‘can’t’.”

So, is it better to set realistic, measurable and achievable goals for oneself? Or is it better to create a grand vision that is obviously way too aggressive, on the understanding that it will not be achieved, but that even a lesser achievement will provide benefit?