In this short time I'm using DOpus I already bumped into RAR files that couldn't be opened with DOpus.
WinRAR 3.40 had no problems with those files.
Also if we would use DOpus solely as a RAR extractor it would be nice to have option to make somekind of icon in file associations and not to use generic icon.
Best wishes, Raisers
Is the problem with single file rar archive, or multi
rar archive?, if the last one, is the first file of the archive
file with the extencion .rar , or some other like .001
or simular, if so I have the solution. For example some
of my rar archives have the first file named with the
extencion .001 , then I have to add that filetype to the
dopus filetypes, and define the function for it, in this case I
selected the MIME type to be:
application/x-rar -compressed
This can be added for other extencions aswell if needed
in the same way, as I found dopus not depacking the
archives if not the first file in the archive are with the
extencion .rar
It will display the content correctly thought.
I hope this solved youre problem, cant come on so much
other with so "short" bug report 
All the best from Helge Brede
I've encountered about 3 rar files so far that have caused Dopus to crash, all contained normal jpg images. Dopus crashed when the rar file was clicked into, content type was enabled to view as thumbs.
When extracted everything behaved fine, perhaps some problem with unrar.dll rather than dopus was my own thoughts. I have deleted the rar files since so cannot check further.
[quote]
HelgeBrede wrote:
Is the problem with single file rar archive, or multi
rar archive[/quote]
Problem was with single file archives. And my dopus also crashed a few times when opening some rar archives.
There is a problem with the current RAR support in Opus where it can pick up a different unrar.dll than the one that comes with Opus.
This happens if another program has installed a different version of unrar.dll into a folder in the path, such as C:\Windows\System32.
I believe it's being addressed in an update that will be available quite soon but until then it's worth searching your drives for extra copies of unrar.dll. It might not be a good idea to delete them, since another program might need them there, but it may explain the problem and you can try renaming them temporarily to see if it fixes it.
If you still have problems the best thing to do is submit a formal bug report via the Opus website and send the RAR in question so it can be checked out.
(Insert standard rant here about how RAR is a completely closed format whose owners seem uninterested in licencing its use in other programs -- except for the limited and badly designed, read-only unrar.dll -- and want everyone to use their WinRAR, which won't plugin to anything. 
)