Summary: This lens just does not get enough respect. I have waffled between this lens and a canon 300 2.8 v1. Finally decided to give this a go because of the used price. Initially had about the same results as the SLRgear review which is ok but not in the Canon league.
Sigma 300mm f/2.8 EX DG HSM APO review by gary1952
28 September 2013
Summary: I own this lens in the Pentax mount and used it for about two years. I then moved to Canon and bought the 300 IS and now the Caono 300 II lens. First I am comparing against the Canon lenses. The Canon 300 II has no equal IMO. Rarely use the Sigma lens now. It is a good lens. Without a T.C.
Pros: Small, compact, easy to use, fairly sharp
Cons: No extras what so ever. Mine is for Pentax and screw AF
Summary: I bought this lens primarily for sports photography and in no way has it disappointed. I had been using my Canon 100-400 but it was jus too slow, though I like the reach it afforded me for times when the Canon 70-200 f/2.8 just wasn't long enough.
One of the best 300 mm F2.8 Prime Lenses for the money
salty dawg, Amazon
13 September 2012
Summary: I have had this lens (in Canon) for over 10 years and it still works flawlessly - I can honestly say this lens performs with the best in class at any price including Canon - the only Cons are it's BIG AND HEAVY - due to it's size it's AF motor is not as fast as a smaller lens - also no external...
Summary: I purchased this lens after renting the Sony G 300 f2.8. Only after reading the reviews of this lens and owning other Sigma EX lenses that I have been very happy using. I noticed that the autofocus was pretty noisy and a bit slower than the Sony lens.
Summary: I have bought a copy of this lens for a pentax k-20 and k-7. It is the worst lens i have in my collection. The international warrenty is completly useless. Do not travel and buy this lens. I have had it for three months and it is a good paper weight.
Summary: I wanted more reach for wildlife photography wanted so I settled on the this lens for use with my Nikon D90. I find it to be sharp even with the 2X teleconverter. I use it with a monopod mostly except for birds in flight.