I have the 24-70 and a number of primes, none of which have IS. For pictures, I don't really miss it, but I wouldn't mind having it, as long as it didn't significantly increase the price or reduce the optical quality.
Occasionally, I shoot video and that's where I wish I had IS. I could use a tri-pod, but the convenience of traveling light has it's appeal. I often find myself swapping a better lens for the 18-200, when shooting video, just for the sake of having the IS.