Having ridden over 100k miles, and replaced a few chains over the years, I can definitely say that o-ring chains last longer (at least on ICE-bikes). They do impart a bit of rolling resistance, as evidenced by brammobrian's comments.
A good o-ring chain, IF CLEANED AND LUBED CORRECTLY, and as often as they're supposed to be, will easily last 15-20,000 miles. If they aren't, they can go bad in as few as 5,000 miles. Remember, ALWAYS replace the chain and sprockets as a set. If you don't, you'll see significant reduction in chain life, as the worn teeth on your sprockets rapidly wear out the new chain.
thus ends my soapbox moment...
the webman
aka eric