I've never used a camera with a full-frame sensor, so I can't personally quantify it's benefits over the APS-C sensor. However I suspect that although the various reviews cite that it offers advantages over the APS-C format, we may well find that to extract the optimum quality from this, (specifically in relation to wide and ultra-wide lenses) a range of reformulated (Telecentric?) optics using a new larger diameter bayonet mount will be eventually needed.

I also have the sneaking feeling the main reason a lot of photographers in the past wanted a 35mm format sensor, was so they can use their existing lenses, because they begrudge shelling out their hard-earned cash on new ones!

The APS-C sensor, evades this thorny issue of a new lens mount, and since the sensor only uses about 3/4 of the Angle of View (AoV) used by the 35mm film format, unwanted artefacts like fringing etc at the edges of the optics FoV do not show up. What would seem to back up this guess of mine, is the fact that it's only been recently that ultra-wide optics have begun to appear for the APS-C format, whilst Olympus have been selling their fast, ultra-wide 7-14mm zoom, (= a 14-28mm on the 35mm format) since 2005.