I think that it can depend on the food, personally.
For instance-reduced fat peanut butter-BAD, BAD, BAD!!! The reason being, is that nuts naturally contain fats-but they are also nutritious, and not the same kind of fats found in potato chips and french fries. The way companies make "reduced fat" peanut butter is they take out some of the peanuts (and some of the nutritional value) and place sugar and other things in its place. So, you may end up with 2-3 grams less fat-but also less nutrition and more sugar and additives. Natural peanut butter is the way to go...no sugar added, no salt added, just peanuts pretty much.
As far as chips are concerned-lower fat is okay/better if they are "baked" as in pretzels or baked Lays or something. Definitely not health food, but better than the fried original versions. Now-the low fat or fat free chips that are actually fried, but fried in Olestra or an oil that is "undigestable" by the body...not so good. (Wow! chips and fat free Pringles are an example here...) They can cause...umm...many trips to the toilet for those with sensitive tummies.
As far as pastries and cookies and bread type things...such as Snackwells, Low Fat Hostess, fat free ice cream and all that-I think the thing to compare is calories. If a regular cookie has 100 calories, and a fat free one has 80, there isn't all that much difference. If there is a 100 calorie difference between items then you might want to enjoy the low fat variety to save the calories. For instance-regular and low fat Pop-Tarts only have 20 calories difference-not enough, in my opinion to make the change.
They are both non-nutritious, and should both be enjoyed sparingly as it is-so when there is hardly a calorie difference then that is your call. On an empty stomach though, as in a snack-a lot of people actually find the full fat version is more satisfying and stays with them longer-low fat or fat free sweets have no fat to keep the digestive sytem busy, so it can be gone (and you starving) before you know it.
I think the best fat free and low fat foods are in nature of course, fruits and vegetables, legumes, rice, oatmeal, etc. If you eat leaner cuts of meat and fish and bake or grill rather than fry, then you are naturally eating a lower fat diet. All of the low fat/fat free products out there mainly only account for treat/snacky type foods anyway...so it is like choosing the lesser of two evils. Neither are really "healthy" so they should be used in moderation.