Well, I started with an immersion chiller when I was doing partial boil extract brews and was not happy with the time it was taking to get through the critical temp range. I added a second immersion chiller as pre-chiller in ice water and agree that it's not real effective. (I didn't try the rock salt in the ice trick though.)
I then moved on to a plate chiller for full boils and I was happy with that, with the exception of the hottest month or so in the summer, until I started researching lagers. I read the Zymurgy article on Jamil's whirlpool/immersion chilling system,described here:
http://www.mrmalty.com/chiller.php
It is beginning to look, to me anyway, that the whrilpool/immersion system may not only be an aid in making better lagers, but ales as well.
So, I'm shopping for a pump and designing designing a hybrid system that will pull wort from the kettle, pump it through the plate chiller and whrilpool it back into the kettle, which will have the immersion chiller running in it. Overkill? Maybe, I'll just have to wait and see how the beers turn out.