We held firm to the NO CUSTOMIZATION rule, we re-engineered our processes to fit SAP. Other than a few hiccups when integrating all aspects of a company that has 100 sub companies, and is under federal, state and local regulation the project basically went off without a hitch.
They are still taking upgrades 6 years later, without issue.
Most companies think they are unique, and special, and feel justified in needed to customize, all those companies are wrong.