I took them for about a month. Can't say they helped with weight loss, but they did help with energy. In the end, it was a waste of money and a pain in the behind to get them.
There are better methods for effective weight loss.
Yes, we would help you over there - but we can help you here too! B12 shots are not a waste of time, but they are not some miracle answer either. They increase your metabolism slightly and give you some extra energy. I've taken them and definitely felt a good boost, but I don't take them anymore. I've found a supplement works well too. Specifically, I haven't tried GNC, so I have no info on that. The main thing is, once you take it - DO SOMETHING like workout with the extra energy or it goes to waste. Good Luck!
My only advice is that its a waste of money.
There is another forum on Med Help, called Weight Loss Alternatives, where people discuss things like this. I'm sure they could help you over there.