The biggest downside or disadvantage of being immortal is that while you keep on enjoying the pleasures life has to offer, your loved ones and friends will perish before you leaving behind a lifetime of memories and regrets. Even if you live on to be more than 100 years of age you have had to witness deaths of certain members of your community and family. It will cause you to lose your empathy for others. Even if you keep on having new experiences you will not enjoy the same thrill as you did with your mates as you will start finding these new people very amateur to everything. You will get frustrated by the repeated drama of human emotions- good or bad. In the end you would stop valuing a human life. Immortality can be more of a curse than blessing.
But if given to a more optimistic person having ideal values of living a life of immortality can also cause wonders. The person having all the time in the world can master a new skill one after the other. He/She can travel all around the world. See every mountain peak, wrestle a lion, dive deep into the oceans of the world. If determined can reach the ultimate form of a human being having extreme intelligence, skill, personality and physical strength.