Caffeine is a pretty harmless substance, but everything needs moderation. The main negative thing that caffeine can do is ruin your natural circadian rhythm which is very important for optimal sleep. Caffeine has a half- life of about 4-6 hours. So if you consume 260 mg of caffeine (tall starbucks coffee) at 3 pm, you'll probably still have 130 mg of caffeine (regular cup of coffee) in your system around 7-9pm.
The reason you don't want this is because caffeine stimulates the sympathetic nervous system. Your body acts as if it is in danger and stays very alert. This is not the state that we want to be in when we are about to go to sleep. The hormone that leads this reaction is called cortisol which helps us wake up in the morning and then it decreases throughout the day. We want our cortisol levels to be low when it's night time.
To wrap up my point, don't consume caffeine late in the day if you want optimal sleep. Regardless of your tolerance, it can still negatively effect the quality of your sleep.
On the other hand, caffeine is a great tool for alertness. Both coffee and green tea have essential nutrients. Studies have found that there's potential that it can decrease the risk of cardiovascular disease, Parkinson's, Type 2 Diabetes, gout, and uterine and liver cancer.
I personally feel like if you use caffeine in moderation without any additives, it could have positive effects. But as always, listen to your body. If you're seeing some negative side effects, then stop or decrease how much you drink. If you notice that you get a big cognitive boost with no negative effects, then go enjoy some caffeine.