Tai Chi can rule…or it can suck. Only about 1/10th of the teachers I’ve seen teach it properly. Practiced properly, it teaches balance, coordination, improved range of movement, focus, and I’d say really improves your muscle control (that’s sort of under coordination, I guess). However, the way most people teach it, it’s only good for maybe relaxation and balance. Tai Chi is a martial art, and to really take advantage of its ability to improve your athletic performance, you should take it from a teacher who has this goal in mind. Your best bet would be to find a teacher of Chen Tai Chi, because I’ve found that style has most of its martial lineage intact with most teachers. As an example, Bolo Yeung, that hyooge chinese guy who was in Enter the Dragon, Bloodsport, and Double Impact, uses Tai Chi as his main martial art.
Of course, if you really want to be able to fight efficiently in the least amount of time, brazilian jiu jitsu, catch wrestling, or sombo/samoz, mixed with some quality boxing training, is a much better choice.