Proxy Pilot - If you scroll your way through these 111 threads you will see a lot written about training bonds:
http://www.pprune.org/search.php?searchid=6278562
But the bottom line is that if you have signed it then you have signed a contract. Other than under exceptional circumstances a person who breaks a bond has demonstrated that they are unreliable, dishonest, lack foresight and don't keep their word, not qualities most employers are looking for, add to that they are scarred for life within the industry, sooner or later it will almost always come back to bite them.
Don't be fooled by people who want to tell you that a decent employer doesn't need to bond, the truth is that bonding has come about because of people who have taken the training then taken off without giving a proper return of service.
I have worked for an airline that treated bonds very, very seriously and bond breakers were ruthlessly and tirelessly pursued, on the other hand, in the same company I have seen people go to the office and negotiate, often coming away very pleasantly surprised at the terms and amount of repayment. Honesty pays.