None of the OTC products are going to really be all that effective. They MAY take you down one or two shades at the most. But after you quit using them, the discoloring will come right back if you smoke or drink coffee, tea and red wine.
Your best bet is to get the whitening trays at your dentist. I got mine for $250 and they work far better than what you can buy at the drugstore. You only have to use the trays for 2 weeks and then just touch them up like once a month to maintain.
OTC strips are an okay option if you don't have the money and just want a slight improvement. But if you really want to see a difference, pay for the trays or get the bleaching (even more expensive though).