Low fat, no fat... what are the benefits?

You definitely have to read the labels.
Some substitutions have no value at all.* For example, fat free muffins are higher in calories than regular muffins.* They just take fat out and put sugar in.

Some substitutions are really good however.

Replacing your half & half or creamer with a no/low fat substitute can save a lot of calories.
Replacing your butter and whole milk with skim milk and a trans-fat free margarine will save some calories and cut way back on saturated fats.
Using nutrasweet/saccharin instead of sugar will save a lot of calories too (in your coffee, tea,..etc)
Diet sodas are way better than the regular ones too.

Good luck.