I use olive oil in most of my recipes in preference to butter or other vegetable oils, except for frying. Olive oil seems to give most foods a nicer flavour, although I don't actually like olives themselves.
The USDA doesn't legally recognize olive oil grades (yet?), but 95% of the worlds production is from IOOC participating countries, which means the legally recognize official designations for olive oil. Extra virgin olive oils come from the first pressing, contain no more than .8% acidity, are judged to have the best taste, and are forbidden from containing chemically refined or treated oil. Virgin olive oil has less than 2% acidity, the next best flavour, and also can't contain chemically refined or treated oil. Olive oil without either label must be less than 1% acidity, and may contain chemically refined and treated oil (usually mixed with some virgin olive oil).
As with most foods lately, I've found that Trader Joe's sells a nice olive oil. I've been using their Sicilian Selezione Extra Virgin Olive Oil, and it does have a good flavour.
Note that generally it is frowned upon to bake or fry foods in olive oil, as the extreme temperature destroys the taste of the oil. I'm not aware of any usage concerns other than that, so I use it for most everything else. It is said to be much better than butter for sautéing, as the milk solids in regular butter burn too easily.