I run a Abus Granit X-plus.
I run a Abus Granit X-plus. heavy as anything but I bung it my satchel when i know I need to lock my bike somewhere. Fortunately i have a fairly safe place to lock my bike at uni and just leave my lock there when im not using it.
Any lock can be broken with enough time and will power (or power tools) the key is to make it too much trouble to be worth the theifs while, or to be more difficult than the bike its next to.
Id recommend paying as much as you can for your lock. The benifit of avoiding the hassel for getting a bike stolen is well worth £50 of investment in my book.
Plus if you have a heavy lock you can think of it as resistance training.