Any one here having tested both the new 300mm F2.8 L IS II and the new Sigma 120-300mm f/2.8 AF APO EX DG OS HSM and having an opinion on how these two said lenses compare in sharpness and general (real world) usability ... Some reviews Rate the sigma as sharper as the Canon 70-200mm F2.8 IS II so it should be really sharp ...
Yes the Sigma is sharp, but not quite as sharp as the the Canon prime. But there are a few things you should be aware of the Sigma 120-300 f2.8 OS which rarely comes up.
1) it's AF is not in the same league as the ef 300mm f2.8 LIS or 70-200 f2.8 LIS II lenses, not even close. It's slower, less accurate and more ponderous. It often locks on and then looses AF just as quick. AI servo with this lens using 61point AF on a 5DIII was particaulrly bad, as was 19 point AI servo on a 7D. Add a teleconverter and this gets a lot worse.
2) The lens drops focal length dramatically as the focus point gets closer to Min Focus distance. While this may not appear to be an issue, it is if you are trying to blur the background using close focus and wide open aperture. In this respect, the Canon prime is way way better, great for animal or people portraits. From my findings, the Sigma drops to about 240mm at 5 meters...which gives worse results to the 70-200 f2.8 at 1.5m at 200mm. In short, both the 70-200 II L and 300 f2.8 LIS are better at melting background and makes my wonder if the Sigma is worth the effort.
3) The Sigma is a lot heavier and larger than it needs to be. Don't confuse weight / mass with quality! The hood is terrible and the tripod ring isn't anyhwere near to the engineering of the Canon versions.