I purchased this lens for the occasional wide angle needs, especially at 24mm or wider indoors. Focusing in low light is an issue unless you have the AF assist light of the 550EX/420EX.
I did not expect much from a lens costing less than half as much as some of my filters. It has however delivered pictures I would otherwise have missed. On serious commisioned work requiring a wide field of view, I rent the exotic "L" series.