There are so many benefits of tai chi, not only health wise but self defense wise too. Here I'll be focusing on the health tai chi benefits, but if you're interested in the self defense side then please go to that section in articles at the top of this page.
Also see Tai Chi for Health for more on Tai Chi and Health
As you probably thought, Tai Chi not only has physical health benefits but also has internal energy and mind benefits. In the beginning of your Tai Chi training you will experience more physical benefits such as fitness, balance, coordination and timing.
After a few months you will start to experience more benefits of Tai Chi such as being more relaxed, bringing your stress levels down and you may even by this point start to feel some balancing out of your energy and a much higher flow of energy, some people experience this as a slight buzzing in the hands or fingers. A balancing of the mind can come out as having a greater sense of focus, sense of purpose and a higher sense of enthusiasm to get things done in general.
As your training progresses you will continue to gain internal mind and energy Tai Chi benefits, everyone will experience these benefits in a different way.
People with Diabetes won't have to take as much medication as the Tai Chi helps to keep blood sugar levels in check.
People recovering from strokes will gain back movement much quicker sometimes gaining movement back that otherwise would not be possible.
People that are overweight or under weight will find it much easier to reach a healthy weight.
These benefits are only the beginning, and will happen in your first year of training if in combination with a healthy diet and healthy lifestyle.
But if you continue to keep going up to the higher levels of Tai Chi the list of Tai Chi benefits are endless, I say to most people (that ask what they can expect) that because Tai Chi helps to balance your body out physically and mentally it will help with almost any ailment in the body and you will feel better because of it. Now if you don't have any ailments and already have a good state of health, the potential for health and fitness is almost endless.
I will start to add other people's experiences with Tai Chi benefits here, as there are so many of them and so many different true stories to be told.