I've looked into objectify a bit and wrote an initial post about it at http://borglin.net/gwt-project/ . There are actually at least 5 different low-level wrapper frameworks at this point (objectify, Twig, SimpleDS, siena, slim3).
I'm currently working on a interview-style comparison between objectify, Twig and SimpleDS together with the authors for each framework. They will answer a bunch of technical questions and I will do some code scenarios with each framework.
In the mean time, you might want to check out these threads:
http://groups.google.com/group/google-appengine-java/tree/browse_frm/thread/4467986eaf01788b/c6d007863a616a1b
http://groups.google.com/group/google-appengine-java/browse_thread/thread/f20d922ffecb310c
http://groups.google.com/group/google-appengine-java/browse_thread/thread/79078132130a3dfe#