I also recently acquired this objective for my new 70D.
Out of the box, without pixel peeping, it seemed OK. However, upon close inspection of pictures taken it displayed considerable front focusing across the zoom range and at all subject distances...
I bought it together with the Dock so I did some quick calibration using the chart found here:http://www.bobatkins.com/photography/technical/focus_testing.html
Using a tripod of course and all other precautions, I tweaked the on camera MFA value until the image was perfectly focused for each focal length and subject distance (except for infinity). I thus checked 18, 20, 24, 28 and 35 at 0.28, 0.35 and 0.5 m, writing down MFA values for each.
I next fed these values into the Sigma Dock software and taking random pictures around the house, the objective seems pretty damned good for close range work now. Probably the MFA values could use some additional tweaking since I am an amateur, I do not have a well lit studio to create the perfect conditions for lens calibration but it is certainly good enough and for critical things there's always LiveView (which is superfast on the 70D anyway).
However, I then set out to test infinity... Having no large studio, I set out to just take pictures of license plates on the parked cars outside. I compared live view shots (using the zoom function there) with what I could get using the viewfinder.
This turned out to indicate massive from focus at all focal lengths.
Using the dock again, I fed values in the +15 range for all focal lengts at infinity. This drastically improved performance using the view finder on far away objects. However, there is a catch: the sigma dock has following subject distances in the correction table: 0.28, 0.35, 0.5 and finally infinity.
I found that, setting rather large MFA for infinity resulted in noticable influence for shots taken at 1 to 2 meters. If the MFA is high enough to get sharp long range shots, the shots at 1-2 meters would now be back focused, particularly at 35mm :s
I therefore resorted to dailing back the MFA at infinity for 35mm and using the on body MFA if I want to take long range shots to add some additional + correction. Like that I seem to get pretty good pictures most of the time but it certainly requires thinking and the objective sometimes still misses in low light. I guess there might be occasions where I miss a more candid shot because of all this.
From what I can tell, the objective is really nice, it certainly merits the rave reviews it is getting, it is really that sharp when in focus. Most people might not even notice slight focus inconsistencies but I m probably a stickler for details so I wanted it spot on and getting it there seems like a bit of a challenge... I think you cannot buy this thing without the dock.
By the way, for setting good infinity MFA values, what would any of you recommend? Is there a more reliable way than what I did? Focusing with viewfinder at 18mm on long range details is a bit tricky anyway because things get a bit tiny so it might be hard to tell if a soft image in these conditions is caused by poor technique or a bad setting...