I did crest whitestrips premium a couple of years ago, and have continued to use them (probably about one pack every couple of months). They hurt my teeth if I do them twice a day for the straight 7 days, so I just do one strip a day for five or six days, then do them sporadically -- once or twice a week.
Anyway, my teeth had been getting kind of dark (not too bad, but I could notice) before this -- too much diet coke and red wine
But since doing the strips I have had numerous people ask if I've had them professionally whitened. Even my dentist and hygenist were impressed! I went from having rather dull teeth to getting loads of compliments on my smile
So they may not work for everyone, but I've found them to be convenient, relatively painless -- like I said, don't try to do them everyday if you have sensitive teeth -- and less expensive than professional stuff. Although, when you buy a thirty dollar pack every two months, it's still somewhat of an investment...but fifteen bucks a month for white teeth seems worth it!