One of the major benefits of OpenERP is that you can easily build extensions for it. Being an Open Source system, we get access to the source code and can modifying it or adopt it to satisfy any requirements the client might have.
Though OpenERP has over 1,500 modules to choose from, at the time of writing, we find that each business is unique and most often than not we need to build an extension to improve that business productivity. The good news is that the cost to build those extensions is way less than doing it for any other system because you can always adopt other modules and change them to suit your requirements. In other words, the starting point to extend OpenERP is way ahead compared to other software, saving you time and money.