First off, the market position: the EOS 6D sits in a new niche that's been carved in the middle of Canon's DSLR line-up.
It's better than <i>almost</i> all the company's APS-C 'crop' digital SLRs (1100D/550D/600D/650D/60D) but is a more basic version of the 5D Mark III and 1D X professional full-frame cameras.
It's better in several ways than the 7D, but worse in others. It's better than the 5D Mark III in some areas, but worse in others. It's better than the aging 5D Mark III in almost all areas, but not <i>all</i> of them.