Totally healthy in moderate amounts. Studies have shown it reduces your bloodsugar and can help prevent diabetes. If you take it within 45 minutes to an hour of your workout it can also enhance performance by mobilizing fat stores and delaying fatigue. One of my professors is doing her research right now on the benefits of caffeine in a healthy diet (she's a PhD in clinical nutrition and a RD)
Just keep it under 400mg a day and make sure you replace the water you lose, as others have mentioned.

And obviously, if you put other stuff in your coffee take that into account in terms of calories. I'm lucky I like my coffee black or with almond milk!