I've shot with a pair of them on my brother's Canons. Wonderfully sharp lens, requiring someone with a decent upper body to shoot for any reasonable length of time handheld. It also stays pretty remarkably sharp with a 1.7-2x converter.
It's at/near the top of the list as my next glass investment, though the 150-600mm Sport lens may have toppled it. I'm hoping to get to the PhotoPlus show in NYC on Saturday to see the new zoom. I'm not one to shoot with a monopod a lot, but I suspect that would change with the 120-300mm. My question becomes, is the sharpness
that much better than the 150-600mm Sport that it justifies 2x the price with the converters I'd want/need?
What I love about it is its ability to focus calibrate at 4 points across the zoom range. RocketCowboy (I think it's him) will tell you that it's because they make them so poorly it requires is, but hell, I've had to calibrate
every zoom I have, regardless of manufacture, and I have to compromise by doing it at only one point in the zoom range, so I scoff at the conjecture.
My brother had some issues with the initial focus calibration of the latest version with his Canon 1D Mk III & IV, and he blogged the snot out of it (if you think I'm obsessive, I'm the easy going guy in the family). Sigma has since fixed the firmware (it was an issue with the Canon firmware messing with the zoom on the 1D only), but there's a lot of great information in there on what the lens is capable of.
Saga of the Sigma 120-300 "Sports"