Choosing lenses can be risky without the luxury of borrowing or renting them to try them out. Even when I try a lens for a while, I just don't have the ability to put it through the rigors that others manage to do when they post their reviews online.
For that reason, I rely on two websites whenever I consider purchasing a lens for my Canon bodies.
A site that's almost encyclopedic in the extent of its reviews is The-Digital-Picture. There's a lot more there than just reviews of Canon lenses, but their reviews of these and of some third party knock-offs are the biggest part of that site's content. It holds nothing back, and based on what they've said I've avoided some Canon prime lenses I otherwise would have purchased.
After checking in with The-Digital-Picture, I always go to the the product reviews section of the Luminous Landscape website. It's not as all-inclusive of Canon lenses as The-Digital-Picture, but if the lense is reviewed there, I'll trust the judgment of the reviewer (often Michael Reichmann) completely.
As a last resort, if the lens I'm considering isn't listed in either of these two sites, I check the online presence of Popular Photography at popphoto.com to see if they have any impressions of the product. I'm always a little skeptical about using it as a source since it depends so much on manufacturers for advertising revenue. I can't help but think there might be a little bias toward giving a positive review of a product that is responsible for thousands of dollars worth of ad space in the magazine.
It's a challenge to make an informed decision, based on objective impressions, when investing in glass. I find the various forums in the web (including reviews on Amazon.com) to be colored by the fact that the reviewers have already spent hundreds of dollars on their lenses before reviewing them, so many of them are likely to be happy with their investment because they've already bought it. That's why I give extra weight to the negative reviews in those forums, and remain mindful of their comments as I make my decision.