Actually, very little good science has been done regarding the alleged "powers" of tai chi. There have been a lot of horribly-designed and pre-biased studies, but very little actual controlled study. What I have seen indicates that, while tai chi can be an effective exercise program, it is not inherently superior to other exercise programs except when high impact or high initial energy requirements are an issue. There is an actual risk specific to tai chi--it seems that chronic knee damage is more common among long-term tai chi practitioners. As for perceptions of "energy" and "alertness", they could easily be a placebo phenomenon. Practitioners expect to "gain energy" and "be more alert", so they are.