Depending upon how you use it, spandex can either make or break your outfit. Spandex is a synthetic fiber that is stronger and more durable than rubber and is often used in the manufacture of clothing. Spandex was invented by DuPont and the name is an anagram of the word "expands," which is exactly what spandex does. Spandex can be very useful to clothing manufacturers because if blended with other materials like cotton, polyester or such, it can be used to make fabrics that stretch. This is particularly desirable for fabrics like denim, which people like to wear tight. While spandex can make your clothing look like it fits you perfectly, it can also make you look cheap and tacky if you don't wear it right.
The biggest mistake you can possibly make with spandex is buying something that does not fit you properly. Do not ever buy spandex that is too small for you. Just because you can get into something does not necessarily mean that it fits. Spandex should be form fitting. It should just barely be hugging your skin. Spandex should absolutely not be puckering or ribbing across you. You should never wear spandex that gives you muffin top or squeezes you so that your fat bulges out from the hems. If you can see through your spandex, you are probably trying to wear it too tight.
The second biggest mistake that people make with spandex is trying to wear undergarments as outerwear. If an item of clothing is mostly spandex, then it is either an undergarment or some kind of sport clothing. The only kinds of pants that are almost all spandex are yoga pants, bicycling shorts or other workout wear. Your leggings and your jeggings are not pants. They are meant to be worn beneath dresses or oversized shirts.