I will buy "sugar free" products if there are NO sweetening agents at all added to them. I have read that artificial sweeteners will cause weight gain just like sugar, and may even start to be metabolized by the body like sugar after a period of time.
If this is true or not, I have no idea. But I would rather just taste the natural flavor of food instead of saturating it with sweetness. I prefer tea, oatmeal, jam, etc. au natural.
As for fat free food, the only difference in taste I've noticed is fat free ice cream. Yuck! But stuff like that is a rare treat for me, so I may as well just go whole hog.