Active TB prevention. The major role of BCG vaccine is TB prevention. Bcg vaccine is also effective for protection against leprosy and disease due to nontuberculous mycobacteria. The efficacy of bcg-protection against active tb- is about 50%, and the duration of protection is believed to be 10 to 15 years.
Always will be PPD+. Apart from the effectiveness debate, those who receive BCG as TB vaccine will always be ppd+ (or be hypersensitive to ppd) which can require imaging such as chest xrays to prove that there is no active disease, when previously a negative PPD would suffice.
Not very good. The BCG vaccine reduces the risk of children developing some of the more severe forms of active tuberculosis. It does not effectively reduce the chance of becoming infected in the first place (which is what most good vaccines do). It is also a live vaccine which means it can have more significant side effects than other vaccines and cannot be given to immune compromised individuals.