i think a few years ago i voted no, but after being such a longtime user of canon material, i must say that having an L lens is really almost necessary.
ease of mind, really.
of course, i do own my fair share of non-L primes that do the same work, but most of the L zooms are of good quality (build and image wise), and it really helps that i can just bring one lens traveling instead of several primes. gets the same sharpness (if not better), and greater focusing speed for grab shots, less CA than on certain primes.
that said, i've started to become an advocator (somewhat) of NOT using non-canon lenses - i do own non-canon lenses (i'm talking about sigma, tokina, etc, rather than the CZ, leica, nikon, etc "adaptor mounted" varieties), but in general performance is really not as good compared to canon lenses.