Here is a blog entry from a Microsoft Dynamics NAV VAR about why 3rd party ISV’s (add-ons) are usually better than custom code. Although written from a NAV point of view, it is also applicable to Microsoft Dynamics GP as well.
Implementation is like marriage. For better or worse, you choose a piece of software, take it under your roof and commit to it for a long term, so help you God. And as in marriage, if you want to live happily ever after with your new software, the my way or the highway attitude doesn’t ….