I think it's against the rules on many sites to use ICM programs during game play, so most people would use it afterward, as a review & study tool anyway. And yes, understanding and applying ICM & FE correctly (at least on a basic level) is essential for succeeding at SnGs.
I'm a Mac user myself and nearly at my wits' end trying to find just one ICM program I can actually run without a virtual machine (with the exception of this
sweet but limited gem), so I rely on extensive notes to make ICM decisions. It becomes intuitive after a while, and whatever it is I'm doing appears sufficient for playing low stakes SnGs profitably, but the difficulty of performing range calculations with any real precision makes me reluctant to move up. I don't understand why no one has tried to monetize on this... is there really that negligible a demand for solid, Mac-compatible ICM software? (Hint to skilled programmers who may be reading this: I would pay)