Sorry but I think they're all a rip off. Have tried a couple and so have some of my friends, they just don't work!
Best way to get white teeth is to get them lightened professionally at the dentist, but it's pricey though. Mind you, some places do it on a payment scheme so you can pay a little amount a month (with a good dosage of interest on top no doubt too!). I've thought about doing this but couldn't justify the money, but if you can afford it, go for it!!