I've been using this pen for a long time now and think it's fantastic!
You can sweat and even swim while using the pen as long as you don't touch your brows. No other pen is as waterproof.
Many complain that it dries out quickly, but remember you can pick off old dried color from the tip and wet it a bit with water to bring it back to life. Also shake it well before use.
The only negative is that the other side (not the liquid color side) sometimes breaks off and is defective.