The first reviews of the C have appeared in the last day. slrgear gives it a good review
http://slrgear.com/reviews/showproduct.php?product=1736
as does ePhotozine
http://www.ephotozine.com/article/sigma-150-600mm-f-5-6-3-dg-os-hsm-contemporary-review-27247
It's very, very good at short focal lengths, better than the Tamron. However, it is weakest at 600mm, where it seems to be weaker than the Tammy. At 403mm, it appears to have dropped to f/6.3.
http://slrgear.com/reviews/showproduct.php?product=1736
as does ePhotozine
http://www.ephotozine.com/article/sigma-150-600mm-f-5-6-3-dg-os-hsm-contemporary-review-27247
It's very, very good at short focal lengths, better than the Tamron. However, it is weakest at 600mm, where it seems to be weaker than the Tammy. At 403mm, it appears to have dropped to f/6.3.