I don't buy fat free anything..but I do use Promise Light(excpet on toast liek another poster said, it's terrible), though I bake with regular butter.
I use Hellmans Light mayo, light sour cream, light cream cheese, light cheese light wheat bread..
I started when I had to do WW years ago, and just kept up with it even after I lost all the baby weight.
One thing that I will only buy the regular version of is peanut butter. I can't even spread the other versions.