Does testosterone therapy really work?

DEAR DOCTOR K: I've seen a lot of commercials advertising testosterone therapy for "low T." Does it live up to its promise? Should I be on it?
DEAR READER: I've seen the commercials, too. They promise that testosterone therapy for low blood levels of testosterone, or "low T," will make you feel more alert, energetic, mentally sharp and sexually functional. Testosterone therapy is a good option for some men, but there are also risks.