Personally I don't feel OpenJPA is mature enough yet. There are other open source libraries that are more mature and I would rather use those. These are the ones I would consider in order:
Hibernate. Hibernate has been around for a long time and has really paved the way for ORM in Java. The only issue I have with Hibernate is the licensing. It is LGPL licensed which may cause some commercial companies to squirm (for reasons I won't go into here). Anyway, if LGPL is an issue for you, it is probably good to steer clear.
EclipseLink. Some background on eclipselink. Toplink Essentials was Oracle's free version of their JPA implementation. EclipseLink was taken from Toplink, Oracle's full blown JPA implementation. EclipseLink is going to be the JPA 2.0 provider for Glassfish v3.0 so it looks like everything is moving away from Toplink Essentials and to EclipseLink. Although the EclipseLink version is only 1.0.2, the product has been around for a long time under other names.
A project I'm working on is on Toplink Essentials right now but we plan to switch to Eclipselink shortly. Hibernate was out for the licensing issues I mentioned earlier.
