Originally Posted by wavetar
There's a lot to 'heart rate' in relation to exercising, as can be seen here:
But to answer the basic question above with a basic answer...target range is best. The keys are keeping it in the range for at least 30 minutes, and consistency over time. As your cardio level improves, you'll notice your heart rate going down given the same amount of running. If the same running pace is only pushing your heart rate around 155 when it used to take 165, time to up the pace a bit.
It's debatable whether pushing beyond the range gives benefits. If it does, it's one of very diminishing returns.
Awesome! Thanks for the answer. I'll start doing that tomorrow with my next run.