I am a happy ISCO 3 lens owner now for about 3 years, but if I had a choice, I would rather not use a lens for the following reasons:
1. Cost!! (these things are expensive!)
2. Reduced lumens when watching 16X9 material (I am way too lazy to move the lens out of the way and I have no room to accommodate a sled).
3. Extra glass is extra glass - the less glass in front of my image, the better.
4. Slight pincushioning - as has been mentioned, there is a slight pincushioning, but I don't notice it on anything but test patterns.
That being said, in my situation I have found the lens to be a wonderful addition. My room is height
limited, but I have all the width that I want, so I built a 2.35:1 screen that used all of my available height. The lens allows me to stretch the width to fill up the entire screen when viewing widescreen material. The degradation from adding the extra glass is minimal with the ISCO 3, as it is a very high quality lens, and my projector puts out enough lumens to offset the loss due to the lens. Also, my projector does not have enough zoom range to use the zoom method of CIH.
The biggest plus about the ISCO 3 is that I have changed projectors 3 times, yet I have not seen any reason to change lenses even once. When you have the best there is (arguably), there is nothing to upgrade to. Also, since the ISCO 3 is a very LARGE lens, it can be used as far as 12" away from the projector lens without any issues, so projectors with recessed lenses can still work with the ISCO 3. In this hobby it has become the single best "investment" I have made, as it is the only piece of equipment I own that has actually appreciated