Questions about Umbrella Insurance

Hi!  My wife and I have a house and two cars, but otherwise nothing particularly risky like a trampoline or an ATV or a boat.  Would it be worthwhile to get an umbrella policy?  How often do events occur that require them, and are there circumstances where the insurance co would refuse to pay?  Our net worth is decent but less than a million.  Thanks for any feedback, and in particular I’d be interested in hearing if anyone has a policy like this that has actually paid out.

http://www.reddit.com/r/Insurance/comments/2bnoas/questions_about_umbrella_insurance/