So finally it's over. Took me 2 years from the mandatory course, learning and taking the exam for Part one, and eventually the assignment and essay, mostly b/c I had a lot of long blocks in between where I totally suspended my work on the certification. If you have the appropriate amount of spare time and motivation, take my advise and try to go trough with the certification path more disciplined and faster than I did, and you probably spend much less time in total.
As for the assignment I had the "Gemstone international" thing and there were not big surprises. It helped me really to actually build at least some parts of the system under design (which is also what I do in my real work). As tools I simply used google docs and pasted screenshots from Enterprise Architect for the UM parts, and later exported the whole thing as HTML, which was most efficient. Took about 4 weeks until I got my results, probably I provided more text and diagrams than atually necessary.
For the the essay questions there were also no big surprises besides what you can read in the well known books and sources. Well, expect that the software crashed short before my time limit ended and left me quite horrified, but fortunately the lady from the test center was able to restore my session on another PC. I think I'm not supposed to reveal much further details. Suffice it to say that I'm quite happy with the result, and that I finally reached the apex of the Oracle Java Certification Path, as I'm also not planning to do any more of the Expert Certification.
Good luck to those who are still in the process - you will do it!