Gimp# fills the niche between scripting languages (easy to write, slow) and C (harder to write, fast). If you need a quick and dirty plug-in where speed doesn't matter that much, write it in any of the scripting languages that come with GIMP (Scheme, Perl, etc.).
Scripting languages are very well fitted for calling existing functionality, shortening manual tasks. You probably don't want pixel manipulation in Scheme. In C, on the other hand, it takes a lot more time to create a plug-in for several reasons: building the GUI is time consuming. Secondly, pixel handling is not completely trivial. You have to know how to traverse through the tiles of an image, etc. Typically the actual algorithm is only a very small (5 - 20 %) part of the total code. GIMP# is not as fast as C, but much faster than a scripting language. Building a decent GUI is much easier than in C.
Changes in this version are:
Enjoy! :-)
Updated December 2, 2010 by prokoudine