The reality is, it depends upon the fat - there are several types;
Trans fat - avoid these like the plague - check the back of box labels. Often, even if something says "trans fat free" it still can contain trans fats as long as they are less then 0.5g per serving....and it only takes 2g of trans fat per day to drastically increase your risk of heart disease.
Monounsaturated/Polyunsaturated fat - fats like olive oil, certain vegetable oils, safflower oil, avocado oil, and many nuts and seeds - these are great for you in moderation - just don't cook with them! Heating these oils causes them to oxidize and go rancid, and cause inflammation and weight GAIN!
Omega 3 fatty acids - a type of polyunsaturated fat which is excellent for you and "essential" because the body can't produce these. Examples would be fats from fatty fish like cod, mackerel, sardines, salmon, trout, herring. Plant sources include flax seed, hemp hearts/seeds, as well as many other nuts and seeds. Chlorella and Spirulina are also some seaweed/vegan based omega 3 fat sources. Omega 3 fats are excellent for helping you lose weight and fight inflammation!
Saturated fat - most of these fats are not very good for you as they raise LDL (the "bad") cholesterol, and can increase your risk of cardiovascular disease and type 2 diabetes. The exception would be coconut oil - it's technically a saturated fat, but (and without getting overly technical) it contains medium chain triglycerides vs. long chain like most other saturated fats.
Because of it's unique properties, coconut oil can provide the necesary fat for critical hormone production in the body (ie. testosterone), cell membranes, as well as raise your basal metabolic rate (the amount of calories you burn while just sitting there!), and can even soothe your digestive system. Also, coconut oil is easily metabolized by the body, used to support muscle, the brain, energy levels and metabolism, so it is not stored as fat. Coconut oil actually lowers syrum cholesterol in the body, and boosts thyroid function! According to Bruce Fife, C.N., N.D. in his book The Coconut Oil Miracle*, the metabolic rate stays elevated for at least 24 hours after eating, meaning more energy and fat burning throughout your day.
In 2009, a study was conducted on women with abdominal obesity and the effects of coconut oil supplementation. Forty women, from age 20 to 40 years who had a waist circumference greater than 34.6 inches, received a daily dietary supplement of either 30 mL of coconut oil or 30 mL of soybean oil. The study lasted 12 weeks. Upon completion of the 12 weeks only the coconut oil group had a reduction in waist circumference.
So how to use coconut oil and how much to use? Firstly, make sure it is cold-processed organic coconut oil (you can even find this at Costco!). Researchers often recommend 3 tablespoons or so spread throughout the day. That seems like a lot, but try adding a tablespoon to your favorite smoothie, and instead of cooking with other oils, cook with coconut oil. It can even replace butter/margarine in recipes as well. It can even be used as a lip moisturizer instead of the petrolium based moisturizers. Or you can do what I do, and use it to make your own chocolate!