"low fat" products are actually not always that good. A lot of them are extremely high in carbs, and will send you on a sugar crash, making you even hungrier. Yeah you don't wanna just eat a big fat sausage roll every day or whatever, but you don't have to go to huge extremes to get the lowest fat stuff possible. A balance is good, and if you want to spend your calorie budget on a bit of full fat butter it won't really do you harm and you should lose weight (provided you aren't abusing it, and measuring the amount etc)
I'd be more likely to check whats actually in the product in terms of rubbish like additives, and i like to avoid "diet food" that has a very high proportion of carbs.
The thing is, low fat foods don't mean low sugar. A lot of weight watchers cakes for example are low fat but higher sugar than their non low fat counter parts. Personally, I don't think you can just follow one in isolation. I think you need to look at all of it, it's no good having a low fat intake if your calories are 3000 a day. At the same time, having 1200 calories a day with a high fat intake or sugar isn't great either. Personally I count my calories and try and keep my fat and saturated fat below a certain amount, avoid anything that's high sugar (unless it's fruit but don't go too crazy with that) and try to hit at least a certain amount of protein each day. Each person is different and will have different views and approaches but I think that that's the best way for me at least.