As the previous thread filled up, here is part 2 of the The Unarchiver thread.
It is much better to post bugs on the bug tracker, rather than in this thread! Anything posted on the bug tracker has much less chance of being forgotten by me!
So, post bugs here: http://code.google.com/p/theunarchiver/issues/list
Also, remember that if you post a bug about a file that is not work, you must also post the file itself, or else there is nothing I can do about it!
I am having the same problem, also on 10.8.2.
Both 3.5 and 3.5.1 will only show Unarchiver preferences when double-clicking an archive. It will not extract the archive unless I then Open the archive.
I deleted the preferences in ~/Library/Containers. This did not help. Preference files are re-created, but archives are not extracted on launch.
Choosing File->Open does still work. I noticed that the sandboxed app told me to ask for permission to extract to my Desktop. After that, quitting The Unarchiver, and double-clicking the archive still does the same thing. It just displays Preferences.
Reverting to 3.4 solves the problem for now.
Also having the issue of only the preferences opening when clicking on an archive - using the newest version