There's a fair amount of flexibility for developing new modules -- more of a necessity for an ERP installation. Every business is different, and while the standard framework is pretty flexible, there's always room for more tables and modified control logic. The development documentation is pretty thick with a fair number of examples. Most of the time is devoted to picking up the structure of the system and the way that the XML files and Java code are turned into tables and forms.
Openbravo doesn't have the same kind of extensive plug-in system as SugarCRM, which limits the way that developers can bundle their features and toss them around. This doesn't mean that the system can't be extended -- it can very easily -- but there's no simple way to pull together a number of enhancements. On the other hand, I've found that plug-in architectures can be a real headache for developers because the plug-ins will eventually find a way to step on each other's toes.
Openbravo's open source community is not as robust as SugarCRM's. There are only 57 projects listed at the company's forge, and many of them are aimed at localizing the language. The wiki, which is much more extensive, is the preferred way to get documentation. They've also done a nice job producing a number of videos, an increasingly common way to tell developers how to get something done.
Openbravo's form system allows you to add new rows to the tables holding data about the clients. The forms can automatically enforce standard rules to prevent important data from being left out.
There are no differences in software features between the community edition and the two commercial editions, but you do get "unlimited bug support" and "installation support" with the paid versions. Openbravo will also sell a hosted version that comes with an Oracle license and a server. I'm guessing that many customers will also look at hiring some of the Openbravo developers to help customize the code. This kind of in-house knowledge is often worth the price.
Compiere ERP and CRM: Form-ed for customization
The Compiere package takes its name from the Italian verb for "accomplish" or "fulfill." It comes with both acronyms, ERP and CRM, indicating that it handles the two jobs pretty well. Like Openbravo, it is a set of database tables built to track customers, products, and the transactions between them. And like SugarCRM it offers a fairly sophisticated set of routines for juggling your customers and reaching out to them.
The Compiere system is built in Java and uses either Oracle or Postgres. Support for any JDBC database is said to be coming eventually. Ports to MySQL, Sybase, and Firebird are said to be in beta now.