Just got my Panny Leica 100-400 the other day and was eager to try it out against my aging, heavy Canon 100-400 (orig mk 1 version). With the Panny on a Panasonic G7, and the Canon on a Canon 70D, looking at both on a table, the Canon only appeared to look about 30% bigger.
Super small & lightweight compared to SLR equivelent lenses - Lightening Fast AF - Sharp Res with excellent Leica color sat - Quick zoom action to go from 100 to 400
Zoom ring a bit stiff when making small adjustments (i.e; changing from 200 to 250mm