There's a few reviews here:
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.fredmiranda.com/reviews/s...page=12&stype=
http://www.luminous-landscape.com/re...n-28-300.shtml
While it's probably a fair bit better than the Sigma (at 8x the price it had best be), I think I'd prefer a
24-105L +
70-300L combo for about the same price, possibly using 2 bodies if changing lenses is a major issue.