I'm thinking of getting the Zoom whitening for $100 from Groupon. I can't even stand the NightWhite gel that one puts overnight in the trays. My teeth are VERY sensitive. I have read 10 or more reviews and 80% of the people that did Zoom said it was painful and they would never do it again. Is it worth the pain for 2 or so days if your teeth are yellow and how much whiter would they REALY get?