We in the Unison group are also about to embark on a major UI redesign
and have been wondering which toolkit to use.  Besides Thorsten's
points, there are two more that are critical concerns for us:

   * seamless portability (Unix and Win32)
   * ability to build statically linked binaries (this is a pretty
     big drawback to the Tk-based solutions)

Comments on these points as well as the others would be very useful. 

       -- Benjamin