Personally, I probably wouldn't use one unless it was lovely blue skies with fluffy clouds (and, as AQ suggests, pic is at 90 degrees to sun) for the saturated blue, rather than reduced glare. Especially as you lose two stops. BUT it won't harm to have one handy and it'll definitely come in useful one day, I guarantee!
As a small note, make sure that if you do use one that you take off polarised sunglasses!