OpenJPA is known as Kodo, which is what BEA bought quite long ago. Kodo was implementation of JDO and now JPA. Also, we use OpenJPA quite intensively.
Thus, I woudn't say that it's not mature. It's fine However I would suggest to use Hibernate, namely, JPA wrappers around Hibernate.
Reason?
1) JPA is very, very similar to Hibernate
2) Many job positions has Hibernate requirement. It's better to lean to mainstream products...basically