As a user of an implementation of JDO specification from Sun (JSR-12), I'd have to say it is quite good. I am a little confused by all of the complaints about it in this thread. Can some one please explain why JDO is an inferior standard as it works very well for me? Has anyone done a real comparison of the two?
As a former Toplink user who watched Toplink prices go through the roof when sold to Webgain, I know the pain of using a proprietary persistence solution. Although Castor is free, I am not sure if I'll receive the same support levels as I have come to expect with Sun JDO where the standard provides me with many alternatives for getting support, training, etc.
Finally, the value of spec is seen in one last place - with where JDO vendors have brought the spec in the 8 months since it was released, it is amazing. The rate of innovation seems to be driven by a common spec and the many vendors (and open source projects) supporting the spec. It appears that Sun's JDO may have been late to the game but is catching up very quickly.