Whats to stop me taking another job once I land, I know the PNP program is employer specific.
With all the paper work done it seems I can take another job without even starting at the employer who gave me a permanent job offer. Is this right or is there something I am overlooking.

Legally, I think you may be right. I can't find anything to the contrary once you have your PR.

Morally however, when an employer have gone to the trouble of sponsoring you, should you not give them some service?

Unless something happens to change your cirsumstances after landing (illness etc), I personally think you should give something back.