Beware your olive oil. According to this fascinating New Yorker article, it’s often adulterated with cheap, rancid vegetable oils for the sake of a quick buck. I’m not sure whether the fridge test discussed by CheeseSlave is reliable. Whole Foods says not — and that they rigorously test their store-label brands for purity. Another alternative is to choose a boutique source like Bariani or Adam’s Ranch.
Personally, I don’t use much olive oil. I’m not much of a salad eater, so I don’t use it for to make dressing. (And yes, I would make my own, as everything store-bought consists largely of industrial vegetable oil.) I use animal fats or coconut oil for cooking. I very much like the flavor they add to foods — and they’re more stable at high heats. From what I’ve read, olive oil ought not be used for high heat cooking. (For more on fats, see this post from Life Spotlight and this detailed article from the Weston A. Price Foundation.)