hehe - i use a f2.8 105mm... but that isnt really telephoto so i cant really comment much - i would imagine that because of the higher performance needed out of the optics of a faster lens the image you get from stopping down a fast lens to f/4 must be better.

But i could be wrong - i am also interested in seeing what people have to say who use these lenses :)

edit - and a faster lens is a godsend for action shots - if you notice my recent series they are all f2 with my canon or 2.8 with my sigma - that made for shutter speeds in the thousandths and completely motion-blur free action shots and it was overcast! ( i was using ISO 400 - which is still very acceptable in terms of noise)

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography