Another soft I use with Linux, without using WINE, is Rawstudio. Results are not as good as with SPP, but quiet acceptable. Ufraw also supports x3f files and is available as a Gimp plugin.
If you're interested, I wrote a perl script that generates thumbnails for the file manager (I actually use Thunar, but it works also for Nautilus). It could also rotate the images very quickly, by writing in the x3f header the desired orientation.
With the latest evolution of Xfce, thumbnailers are not outer scripts anymore, but rely on a dbus protocol. The daemon used to generate thumbnails is called tumbler and need plugins to generate thumbnails: in the sources are included plugins for jpeg, gdk-pixbuf (pnm, tiff, png, etc), pdf, and some more, but unfortunately not for raw files, x3f raw files in particular. Therefore, I write a plugin for tumbler allowing thumbnails for x3f raw files to be generated: the plugin is able to generate thumbnails for all Sigmas cameras (from SD9 to DP2x), rotating them accordingly to the rotation field contained in the x3f header.
Post a reply if you want me to give a link to those utilities...
Regards