I'd be interested to hear what people's favorites are, especially Mike and Merrick
I rank the JPA implementations that I have experience with as follows:
Don't have any experience with other implementations - always wanted to check out Kodo but never got around to it.
I have seen things you people would not believe, attack ships on fire off the shoulder of Orion, c-beams sparkling in the dark near the Tennhauser Gate. All these moments will be lost in time, like tears in the rain.
Kodo started out as a JDO implementation from Solarmetric. Solarmetric was purchased by BEA, which is now owned by Oracle. Somewhere along the line, the decision was made to open-source a good deal of the system, which they did about the point where JDO became JDO 2, then JPA. The open-source code is the basis of the Apache OpenJPA project. You can see just how strong the debt that OpenJPA owes to Kodo if you look at their documentation and see just how strong the resemblance is.
Along the route, two very helpful people have stayed with the platform all the way from its Solarmetric days - Abe White and Patrick Lindsey. They've been very patient with me. Thank goodness.
I used Kodo on a major project several years ago and was very pleased with the results. More recently, I've been using Hibernate JPA, but I still use the OpenJPA tools for reverse-engineering databases and the like. I have an XSLT that can take their schema file and generate boilerplate JSF CRUD pages.
An IDE is no substitute for an Intelligent Developer.