I have a nice opal ring at home. Opals are one of my favorite gems. I'm also partial to Star Sapphires and Alexanderite.
Opals can be very fragile, almost as fragile as pearls, so you have to be careful not to bash them around too much. I don't think you can use regular jewelry cleaner on them either like you do with normal gemstones. They're a (mild!) soap and water sort of gem.
One thing most people look for in an opal is how much "fire" it has, that is, how much color and flair is in the stone. You'll also want to check the color of the stone. Most opals are white or just off white. There's also black opals, but those are rarer and a lot more expensive from what I understand.
Opals are very popular, partly because they are one of October's birthstones (the other is pink sapphire).