The chief health benefit to cranberry juice seems to be a reduction in urinary tract infections (UTIs) in women. There is fairly good evidence of this.
Apparently, cranberry juice contains a substance that inhibits the adherence of a bacterium called E. coli to the bladder wall. E. coli is the commonest cause of conventional UTIs, and if it can't attach properly to the bladder, it is powerless to cause infection (it'll just be washed away next time you urinate).
At present, the research seems to show that cranberry juice can both prevent and even treat a UTI, although we need more studies to confirm this properly.
Cranberry juice is also claimed to treat or prevent several other conditions, but the evidence doesn't support that at the moment (though it may in the future!).
One small note: cranberry juice can interact with medication like warfarin, however, so if you're taking medication and plan on taking lots of cranberry juice, just check quickly with your doctor first! (How? It seems to inhibit cytochrome P450 'detoxification' enzymes in the liver, resulting in an increased half-life for drugs depending on this pathway for elimination.)