There is a new paper out titled, "Effect of Tai Chi vs Aerobic Exercise on Blood Pressure in Patients With Prehypertension: A Randomized Clinical Trial" which shows significant improvements using tai chi compared against an aerobic exercise group.
Other benefits are outlines on the website by University Hospitals
For other sources that do a decent job detailing the benefits of tai chi in clinical studies, I recommend the book, "The Harvard Medical School Guide to Tai Chi".
-dave