I've always liked Bjørn's reviews at http://www.naturfotograf.com/index2_PC.html. Just follow the "lenses" link in the vertical menu on the left. This site will NOT compare them to other manufacturers but will allow you to compare them with each other.
I second billkoe's post above: Bjørn Rørslett's site (http://www.naturfotograf.com/index2.html) is my first choice for lens info, and I have bought a number of the lenses he rates highly and have never disagreed, in particular when it comes to macro lenses.
My second choice is Thom Hogan's site (http://www.bythom.com) for thoughtful reviews.
And for details like close-focus distance, filter size, etc. (not lens suggestions!), I can find what I need most quickly at Ken Rockwell's site (http://www.kenrockwell.com).
