You can only get a preview because production models aren't available yet, so those who are testing are using pre-production models. A last round of beta testing as it were. Once the cameras are commercially available, you'll start to see full reviews.
I like
dpreview, but they do seem to be unduly obsessed with high iso noise performance these days, which sets off a similar hypercriticism in seemingly everybody who reads their reviews.
Rob Galbraith is the guy if you want to know about Canon gear from a pro POV. He exhaustively covered the 1DMkIII's autofocus issues when they arose.
I also love
the Luminous Landscape reviews, since those guys typically shoot medium format and art photography and have decades of experience with a much wider variety of photography equipment than most of the digital-only reviewers out there today. They'll give you more of the feel and gestalt of the thing, than a spec list.