The appointment takes about 20 minutes, and they do a great thorough check of all your moles and other skin irregularities. If you go once a year or so, they'll almost certainly catch anything that needs to be removed before it gets deadly.
Two stories:
First, a close friend of mine ignored a skin irregularity for years and years until he had to have his entire thumb amputated. Unfortunately, the melanoma had already spread, and now he's a terminal patient (although a few clinical trials have helped extend his life beyond the initially predicted timeline!).
Second, my business partner goes in to get his skin checked every year. The dermatologist caught an irregular mole, confirmed it was cancerous, but because of how soon it was caught, the doctor removed it, and he hasn't had any other signs of cancer since.