Due to the, say, failure of JDO (see the public review ballot) some developers (about a thousand) "signed" the Petition to the Java Community Process Executive Committee.
My opinion about JDO in some short statements is:
JDO is transparent in a different way than, for example, java.io.Serializable is. JDO manipulates class files, not the source code. I don't like this as you could run into difficulties when doing Test-Driven Development (TDD), and I like to see what is there (namely in the source code. There sometimes was a post on Joel on Software, The Law of Leaky Abstraction, about the problems with abstractions.).
JDO seems to be a good solution but not the best I could dream of. Just one remark to the disposedness of SAP, a really big software development company, about JDO: In later releases of the NetWeaver Developer Studio for Java, part of the huge NetWeaver architecture, JDO was not supported. Period. You could hack and get JDO out of it, but who wants to develop software that way?
I would prefer a solution based on a code generator, or, if possible, something similar to java.io.Serializable (althought the latter may be too inconcrete to argue about, as it's only about introducing a marker interface). But a code generator would add the necessary flexibility as well as the necessary capability! And you could TDD your code as you like, do code coverage analysis on source code, which would make things much more fun. Some say, don't test third-party products. But relying on "a" third party toolkit made by people not known the to-believed trusting person is not the best sort of risk management to think about. Only one argument: Open-Source. Although not bad in general, many of these projects have kind of a charme like "Ahh, there is someone who wants to contribute, I don't know him, but he is willing to add something, so let him do. And if I, the project admin, find time, I will check what he really has done". I love open source but I would not trust it generally. There is no support, in contrast to many commercial products (allow me to cut it short here).