A.) No bicycle locking system can ever be impregnable so the decision as to how much money one is prepared to invest is what it all boils down to in the end.
B.) Geography and both daytime and overnight location come into play in arriving at the answer to A.
Bicycle parking areas in both Oxford and Cambridge are full of cheap, elderly bikes looking rusty, dirty and completely unloved. Many of those however have decent locks as the owners care more about the bikes actually being where they were left than about the quality and desirability of the machines themselves.
By and large, bike thieves prefer 'name' bikes in half-decent condition that can be easily 'processed' for a bit of profit. High-end bikes in the wrong location are a magnet for the professional thieves and they are particularly adept at releasing locked bikes.
In the home situation, if one has a garden, I have often thought that a secure, large dog kennel area, replete with a couple of rottweilers or similar may be the ideal spot to house expensive bikes!
Tom