According the most recent review by researchers from the Cochrane Library looking at a total of 24 studies and over 4000 patients revealed that taking cranberry juice regularly did NOT reduce nor prevent symptomatic urinary tract infections. The researchers did find however a small reduction in frequency of urinary tract infections for those individuals who get recurrent urinary tract infections.
Cranberry juice contains chemicals called proanthrocyanidins that can prevent bacteria from sticking to the bladder wall. This is felt to be the first step in the infection process.
So although cranberry juice does not seem to help prevent urinary tract infections there is still evidence that it may help to reduce tooth decay, lower the risk of heart disease and lessen stomach ulcers due to these proanthrocyanidins.