A friend of mine bought a vocal instruction CD, and was pleased with it (though he didn't persist with it beyond the novelty phase).
He gave me a taped copy, which I duly had a go with. But to be honest it was really useless compared with having a proper teacher. You need feedback - and its much better if you can SEE and discuss examples rather than just hearing them. Unless you already know what you're doing then you're unlikely to derive much benefit beyond what you'd get by listening to any good singer's regular CD, I reckon (judging by the example I heard).
If there is any possibility of your getting lessons then I'd suggest you do that & not bother with a tape/cd. You can record your lessons to practice with in between whiles - and if you space out your lessons according to your finances then it needn't break the bank.

Kris