Dynamics NAV and AX
Dynamics GP has powerful customisation capability with Modifier, VBA, Dexterity, Extender and now Visual Studio.
Navision (Dynamics NAV) and Axapta (Dynamics AX) have always been renowned for their customisation ability - in fact one could almost consider the products themselves to be a development environment (or toolkit).
This is both good and bad!
Because GP is more of a ready-made solution, it is much faster to implement. In some situations, such as complex manufacturing, then the core flexibility of Navision or Axapta may be an advantage.
This does, however, bring risks to the customer because the development environment is proprietary. The customer typically becomes very reliant upon the implementation partner's skills. Also because the source code dictionary is open it can be easily changed, this often causes problems with reliability and stability. Upgrades become a real issue since most installations end up having a different code base.
GP has multi-language support, multi-currency and local compliance for VAT and Accounting for the main countries in Europe, but because of the history there is a much stronger Navision partner channel available in mainland Europe.
Axapta is much more scalable than Navision and is well suited for complex, multi-national manufacturing groups.
GP is the cheapest to implement, typically Axapta is much more expensive to implement, with consultancy costs two to three times software costs. As a comparison GP implementation consultancy is typically less than the software cost. Navision is somewhere in between these extremes.
GP is strongest for service industries, particularly financial services, public sector, not for profit and distribution.
Navision and Axapta are stronger in manufacturing industries.