For those concerned about the weight, I was too, so I hired one for a motor racing weekend. I used it hand-held for 3 days, panning/tracking at 1/100 to 1/160th for blur, shooting thousands of frames with my 1-DX on burst. I'm a 70kg middle aged weakling, so I felt it, but persevered without untoward after effects.
Suffice to say that I was impressed enough to purchase it off the hire company, 4 months old, cUS$900 under new price. I strongly recommend anyone even remotely interested in this lens hire it and see for yourself.
I now need to get a definitive answer to which 1.4x (Sigma or Canon) will work best with this beast?
You imply that a Canon TC would fit and work? I have a Canon 1.4 TC II and would be very interested in people's experiencies (if any) with a combination of this TC and the Sigma 120-300 f/2.8 DG OS HSM. Both full frame and APS-C results.
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 2.0x III, 70-300L | E-mount: SY12 f/2, 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses