Although it can be good discipline to shot with a single lens I've found my Tamron 17-35mm to be a bit constraining in some situations, especially when needing to blur out a background. The bokeh from the Tamron leaves much to be desired. I've now added a Canon EF 100mm f/2.8 USM Macro to my Canon system. This lens is sharp wide open and has beautiful bokeh.
Read the lens reviews here