Do you really need the fastest L-series lens on the block? Do you even need an L-series lens at all? Well, yes and no, I think it depends on your specific application.

For low illumination applications, if flash photography is either not allowed or undesirable, and depending on your camera's high ISO performance a fast L lens may be the only possible solution. On the other hand, if normal outdoor illumination is your typical application then maybe you can save a few bucks.

I've always been impressed with my copy of the 28-135 lens; if you look carefully you will see that at f/5.6 it performs pretty darn well at 100 mm compared to the the higher cost alternatives used in this comparison.