From wikipedia, backed up with sources:
"A study performed at the University of Birmingham showed that average fat oxidation rates were 17% higher after ingestion of green tea extract than after ingestion of a placebo.[17] Likewise, the contribution of fat oxidation to total energy expenditure was also significantly higher by a similar percentage following ingestion of green tea extract. This implies that ingestion of green tea extract can not only increase fat oxidation during moderately intensive exercise but also improve insulin sensitivity and glucose tolerance in healthy young men."
There is also evidence that as well as burning more fat and boosting insulin sensitivity (so staving off Type 2), it can help to prevent cardiovascular disease and several types of cancer, it can reduce cholesterol, boost the immune system, protect brain cells from Parkinson's and Alzheimer's disease, reduce blood pressure, prevent glaucoma, prevent arthritis, treat skin conditions, and is a powerful antioxidant.
So that's all pretty good.
The bad: excessive, continued consumption of green tea EXTRACT (not the drink) over long periods have been shown to cause liver damage when taken on an empty stomach. So be careful, drink in moderation, and drink after a meal or snack.
The good far outweighs the bad for me, but everyone will make their own minds up
I alternate green tea with Chai Rooibos tea and probably total about 5 cups a day.