This is great, but even more important is the tab at the far right that says "Mark ALL messages as read"
You click on "Show unread posts since last visit.", then you open the ones that interest you, and then you click on "Mark ALL messages as read" and all the boring ones disappear (I find this invaluable for certain dealers and auctions, and also topics I have no interest in, like sports discussions).
Using these two together judiciously lets you view everything you do care about, and never have to see what you don't care about, and makes your time here far more productive.