I have recently upped my shoot to include the use of the Nikon SB-900 with as many shots as I think necessary. And the results versus those shots I took without the flash is beginning to prove that day-light fill flash is just as important as those fired at night.
These wonder lights simply work like a magic wand of some sort, lifting each grain residing on edges of my subjects away almost magically, while giving stronger shadow contrast on those same edges! Conventional lens quality seems to be "pushed up a few stops" with each use of these speedlights. I am impressed...really.
So, if you have, all along, been using the built-in flash for fill flashes, you ain't see nothin' yet!