Having used the lens for a while now, I thought I'd throw some more observations in here. Not enough for a full review which I'll do some day after I finish one for the 100-400L...
The zoom action was quite stiff when I got it, so much so it didn't need the zoom lock. It has loosened up and now it will extend by gravity if left dangling while walking. No other problems there.
Sometimes I get a lens error reported by the body. This has happened more than once, and only with this lens. I don't know if there might be a loose connection or maybe the lens firmware isn't quite perfect. But it is very rare, happening somewhere around 3 to 5 times since I got it. On each occasion I just turned off the camera, twist the lens out and back to force the contact to remake, and turn camera back on again. Then all is fine.
Compared to other lenses I have used, this is the strongest stabilisation I've seen, although the only comparisons I have are my 100-400L and a borrowed 24-105L.
Sharpness is decent at 18mm end where I would compare it as similar or maybe even better than the Canon 17-40L. But I don't feel satisfied at the 270mm end where it always feels soft. I tend to use the wide end mostly for landscapes, in part as this is my only wide angle Canon fit lens. There is some distortion and CA but I don't find them a problem in practice.
When used for IR, it shows slight signs of a centre hotspot.
As the maximum magnification is at 270mm, "macro" shots by it also suffer from the softness.
I'm in a love hate relationship with this lens. If I had to pick one lens to go into the unknown, it will be this, but I know I'll be a little disappointed knowing I could do better at the long end.
_________________Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.