This is a highly arguable subject, but the fact is that any lock can and has been defeated by various methods. If you have a $500 bike there is no need to use $500 in locks, however you may reconsider that if you have a $5,000 plus bike. The best defense against theft is to have two bikes, a junker you lock up in areas you would be worried about parking the good one, and save the good one for training and racing if that's what you do. And lock theft warranties are rarely paid out on! So don't expect those to work for you either. And the cops could care less about your bike, though they care more if someone steals a $1 new Pic pen out of a store then they do about your $10,000 bike!! Go figure.
If you must lock up an expensive bike and have no desire to lock up a junker instead then get two locks not one. Get a really nice D or some call U lock and a thick armored cable lock. Two locks will require two different tools most of the time. And here is the best way to use a U lock; see:
http://www.missinglink.org/page/how-lock-bike Then take the second cable lock and run it through the same way as the U lock. Or instead of a armored cable lock there is a lock called the Master Lock Street Culfs, or if price is no object there's an even better foldable lock made by Abus called the Bordo Granit X-Plus 6500 but that thing is expensive.