I installed it(Poseidon1.1) yesterday itself and only words I can think of are "I AM IMPRESSED".
About reverse engg., I tried generating class diagrams for my existing project and it did a neat job.
At present it is able to reverse engineer from "*.java" files and not .jar or .class but one can work around that problem very easily.
You should really try it out.
Regards,
Bhushan