1) Good short read on metal, chains etc:
http://lockitt.com/chaininfo.htm


On that video:
Those are 1 meter long strong cutters. They are also leaned against the floor to cut that Abus chain. Thieves don't cary 1 meter long cutters underneath their jaskets. Those cutters would cut most U-locks as well.

Tips:
Try to lock both wheels to the frame and frame to something immobile, not easily cut. This can be done with two chains, or one long chain, chain plus u-lock, whatever you like.

Make sure chain/u-lock isn't close to the ground, so thieves can not use the ground as leverage, or use hammer/chisel to break locks/chains.

Make sure both the chain, lock and key mechanism are good. Chain is as good as it's weakest link. If you can afford, Abus stuff that is above protection level 15 (on Abus scale) is good. Don't know about Kryptonite stuff, none available where I live. I would never pay that much for locks, so any other chain that's over 60 rockwell and a decent lock will probably do.

If your bike is apealing enough and if there is enough time, any lock/chain can be broken.