I bought this lens after studying several reviews between and the Tamron. Chose this one because of the availability of the dock even though I did not purchase it originally. I also like the fact that the zoom worked in the same direction as my Canons. The lens was purchased to be used primarily on a Canon 70D and sometimes on a Canon 6D. The lens required MFA to +3 on the 70D at 600MM. No adjustment was required on the 6D.
I was very pleased with the lens when I got it as far as build quality, location of controls and flexibility of controls. Downside is the manual focus ring is very small, but I rarely use manual focus. The included reversible lens hood is lightweight plastic and attaches firmly in place. The included tripod foot is hefty and is removable. The zoom ring is very smooth with locking points at intervals between 150mm and 600mm with the two extremes being hard locks. The lens also came with a very nice soft sided case that looks to be very durable and fits the lens snugly.
Image quality is very good throughout the range of the lens with the shorter end being slightly better. The default settings for OS and focus speed were acceptable, but I felt the OS should be better for the claimed improvement over a non-stabilized lens. I read several comments that said changing the lens from default stabilization to Dynamic stabilization made a big difference. I decide to purchase the dock in order to make this change and to be able to get any firmware updates without sending the lens to Sigma. I changed the OS to Dynamic and the focus to speed priority and was amazed at the difference in OS stability. Handheld shots at 1/100 sec are easily made at 600mm. The stabilization is rock solid when engaged. I have not noticed any degradation in focus accuracy with speed priority and it is fast. Not as fast as my Canon 400mm f5.6 L, but perfectly acceptable. When focusing from extremes of shorter focus distances to longer distances it does require, at times, a little kick start by bumping the manual focus ring. This also occurred with the default setting and has not been a major problem.
The two Custom Settings allow you to program OS and Focus priority for different situations and the lens responds well to these settings. Sigma recommends turning off OS when the lens is used on a tripod but I have not seen any noticeable difference in image quality if it is left on. I have never used the panning only OS mode so cannot comment on its usefulness.
Bottom line is I am extremely pleased with this lens and the quality of the pictures taken with it on both the 70D and 6D. Main uses have been birding/wildlife and girls fast pitch softball with most shots taken from behind the outfield fence. Extremely pleased with its performance as all the softball shots are taken hand held with very good results. The lens is a great value for its cost and the addition of the dock for firmware upgrades and performance changes just adds to the value of the lens.
Cannot think of any major negatives of the lens.