I use the low fat mayo. It has about 1/2 the calories and I've been using it for so long I really don't notice the difference between it and regular mayo.
One of the reasons I do is that my body (and most everyone) needs some fat in my diet. It helps slow down digestion so I feel full longer and gives me the "mouth feel" that tells my brain that I have eaten, so that my hunger is sated quicker.
That being said, I don't use mayo very much. But for an occassional tuna salad or egg salad adding a little mayo seems to make the meal just feel more filling.
2.5 years, 45 pounds later... 128ish pounds