The M2 is smooth, easy to maintain, but not as efficient as others. It would probably be my best all around marker right now. The M2 just feels right in my hands and points very well for me. It just feels right.
The CS1 is more efficient, but not as smooth as my M2, and very easy to maintain. The ergonomics for me are not nearly as good as the M2, but still comfortable.
The ICE also feels great in the hands, it is as smooth as a CS1, but not as efficient nor as easy to maintain. That is not to say that the ICE is hard to maintain as it is not but the CS1 and M2 are easier.
Here is a link to another review I wrote covering many recient high-end markers. I'm going to port each to the review section here at some point.