Have you read Bryan's review? http://www.the-digital-picture.com/R...ns-Review.aspx
My experience with Tamro has been good for the price. The zoom ring rotates opposite the canon so when I switch from one lens to another I get confused on direction. When I need to zoom quickly this is a problem. The canon is faster focusing and sharper. For these reasons I stick pretty much with canon lenses.
Bryan mentioned inconsistent focusing. This is troubling.
Bottom line, you usually can't go wrong with a Tamron but the canon is the better choice.