Nowadays everything has a lite or diet version. Some of these are great. I’m particularly fond of turkey kielbasa. It’s like half the calories of normal and pretty close. OTOH, there are things like baked corn chips, where the calorie improvement is pretty meaningless (10 calories per ounce?) and they aren’t really as good.
So what’s on your good and bad list?
Sugar-Free Jello. Has kind of an artificial taste but at only 10 calories/serving I got over it. Regular Jello is 80 calories/serving, but its almost all sugar.
The Sugar Free Pudding tastes a lot less artificial, but has more calories (from the milk, natch). Still, I think a serving made with skim milk is like 60 calories. Not bad either and a much more satisfying splurge than full-sugar Jello. I like pistachio. 
That said, I would relying too much on “diet” or reduced calorie products. They are a crutch that prevents you from developing permanent good eating habits - namely, lean meats, high fiber veggies, high fiber grains, high nutrition fruits. They are ok for “once in a while” though, IMHO.
I use 2% yogurt.
The regular that I have is like 260 calories for 7 oz. 20g of fat.
The 2% is 130 calories. 7g of fat. That’s a lot of calories over the course of a week.
That’s pretty much it, though.
I tend to like 100 calorie versions of food, where they use portion control instead of artificial sweeteners (though I don’t mind Nutrasweet or Spenda). They are often variations that are like the original in name only (100 calorie Oreos, which are just chocolate wafers without filling), but since they’re made from regular ingredients, they can be quite good. The gold standard are the Entemanns’ 100-calorie chocolate chip cookies, which are as good as any regular cookie out there (except for Freihofer’s of course).
The worst of this category are Thomas’s 100-calorie English muffins. First of all, English muffins are only calories each, so all they had to do was make their regular muffin a little smaller – eight 90 calorie muffins to a pack instead of six 120s. But they kept them the same size and added some bad tasting fiber that leads to an unappetizing taste and consistency.
Baked Lays.
I love Baked Lays and would rather eat them than “normal” potato chips. Even then, at most I have a small bag of them once a month.
I’ve lost my love for regular chips.
That lite syrup is NOT worth it to me. I would rather have a half serving of normal syrup than a full serving of the nasty lite stuff.
I love the lite syrup myself, but it must be the syrup that is not made with splenda or any artificial sweetener. It is the same as normal syrup but a bit thinner and not quite as sweet. It is good stuff and I highly recommend it. I also love Baked Lays and other baked chips. Talk about yummy!
Any kind of diet chocolate is the devil though.
I prefer reduced fat peanut butter to the full fat variety. I find the flavor and texture is enhanced–full fat peanut butter tastes greasy and seems excessively gooey to me now.
Technically it isn’t a “diet” food, since both types have the same number of calories (at least in the Jif brand, which is my basis for comparison). But I guess it is worthwhile if you need to cut down on fat for reasons other than weight control.
I’m okay with most “diet” versions of things.
But fat free cheese sucks.
Okay, low-fat cheese isn’t too bad, but NEVER FAT FREE.
Fat free dairy anything is definitely not worth it. I find it tastes so disgusting I end up tossing it rather than eating it. Ptooey.
I agree that sugar-free Jello is delicious.
I buy no sugar added spaghetti sauce (Hunts makes a canned variety) that is both cheap and good tasting.
Jolly Time has an awesome light kettle corn. It’s either 0 points or 1 point, I forget, and you’d never know.
Diet Dr. Pepper and Diet 7-up are both pretty good.
I’ll usually buy low-fat sour cream instead of full fat, since it’s substantially lower in fat and calories. Besides, since it’s usually mixed into something or spooned on as a condiment, it’s not in a large enough quantity to notice the difference.
Fat-free sour cream is an abomination, though, and will never pass through my lips willingly.
I can’t stand anything made with artificial sweeteners, which annoys me to no end when buying flavoured low-fat yogurt. Apparently someone responsible for stocking the shelves in the local supermarkets refuses to believe that some of us hate the taste of Splenda and aspartame with the burning fire of a thousand suns. 
This also rules out most low-cal beverages… why oh why can’t someone make a flavoured water that isn’t sweetened???
Vitalicious brownies are stellar, and only 100 calories.
Coke Zero is better than real Coke to me. Hansen’s makes good caffeine-free diet Root Beer and Tangerine Lime sodas, too.
I really like the 100-calorie pack version of Hostess chocolate cupcakes. The other flavors aren’t quite as good, though.
I don’t particularly like many low-fat cheeses, but Cabot’s makes a really good 50% light cheddar.
I’m actively watching my weight (and will for the rest of my life). I’m not a huge fan of most “diet” foods because I find them unsatisfying (for example, it would take 5-6 100 calorie snack packs to make me feel full, it’s just “empty” food to me).
I do like the following:
Low fat dairy (yogurt, sour cream, cottage cheese, cheese) - I can do fat free yogurt (Fage 0% Greek yogurt is particularly good, or Horizons fat free vanilla) but fat free cheese just doesn’t taste right or melt right
La Tortilla Factory 50 calorie whole wheat tortillas
Morningstar Farms veggie “crumbles” - I use them for making chili or home-made pasta sauce
Morningstar Farms Spicy Black Bean burger
Annie’s Naturals Gingerly Vinaigrette salad dressing
Aidell’s chicken sausage (particularly habanero and green chile), they also have a great chicken sausage meatball
And of course, my “diet” favorites - lots and lots of fresh fruits and vegetables! 
Paul Newman’s Thai dressing is low calorie and about the only low calorie salad dressing I have ever tried that I like. I usually have the regular stuff, and try to use it sparingly.
For dairy products, I prefer:
Milk - Fat free
Yogurt - Fat free
Cheese, sour cream, cottage cheese - next step up from fat free, i.e. 2% or whatever
Diet frozen dinners are fine, probably just as good as regular ones. Michelina’s are cheap and not bad, Smart Ones are not as cheap but slightly better.
Baked Lay’s (or the Target equivalent, even better) are fine, I prefer them to regular chips in fact.
I prefer to drink only diet sodas or Crystal Light. No juices as they have calories and are sugary. Though, on occasion, I will drink “light” cranberry juice.
I don’t ever get diet bread, it always seems like inedible foam to me - go for 100% whole wheat and you’ll be fine.
For coffee drinks, the sugar-free syrups are all generally OK except for chocolate (which is vile). Caramel is one of the better ones. With skim milk, it’s a pretty safe indulgence.
100 calorie popcorn packs taste fine to me. I like the “Kettle Corn” variety.
For soups, I find most light soups also are “low sodium” and disgusting, so either add salt or go for the Archer Farms (Target brand) vegetable soup which is low-cal and fantastic. For canned chili, vegetarian chili (I like Hormel) is better than any diet version I’ve had.
I pretty much eschew all things “diet”. If they’re lower fat, they’ve compensated by bumping up the sugars and carbs; if they’re lower calorie, they usually have a lot a crappy fillers. After years of eating tons of different diet products, I’ve transitioned to only “real” foods in moderation with lots of fruits and vegetables and maintain my weight (always a struggle) far more easily than I ever have. When I need to drop a few pounds, I add more vegetables before each meal.
I do still drink Diet Coke on the weekends (I drink tea during the week) and have one Hansens Diet Soda each evening. They’re so sweet, they satisfy my craving for something sweet right before sleep.
I like anything made with splenda, especially publix brand no sugar added ice cream. Of course, having reactive hypoglycemia means that sugar is the devil, so I don’t have much of a choice. If splenda didn’t exist, I would go crazy, because all other fake sweeteners are generally, you know, disgusting.
Oh yeah, coffee drinks I don’t eat a lot of “sugar-free” food, but I love no fat lattes with sugar free syrup at coffee shops. Starbucks has recently started marketing “skinny” lattes (no fat, no sugar) and they added a couple of new flavors of sugar free syrup - mocha (which I think tastes okay!) and gingerbread. One of my big changes was realizing that I didn’t have to get the biggest size all the time, I used to always order the venti caramel latte with extra whip. Now, I’m perfectly happy with a tall skinny mocha!
Starbucks definitely has some “diet” foods which are NOT worth it - if you want to get any of their baked goods, carefully check the website. For example, the “no sugar added banana walnut loaf” is 480 calories with 28 grams of fat. I generally get the chocolatey biscotti for a sweet treat - 140 calories!