Its only unhealthy when you make it unhealthy.
Green tea is the king of tea. Its an excellent anit-oxident which flushes out all impurities in your body. Also contains caffeine which will give you the kick of energy that caffeine from coffee will give you. The numbers of caffeine in green tea versus coffee are suppose to be really close, but any herbal of that sort will be healthier to drink. If you can drink it without sugar and milk.. go for it. Small moderations of both will not hurt you. Although careful on your choices of sugar. People drink it to lose weight, gain weight, people said it relaxes you after a good workout, and the more you drink of this.. the better it will be for you.