I like the 3100. It operates in a different manner than the old school autopilots that I am used to, which is okay, it just requires changing your procedures slightly.
The key things to know about the 3100 are:
1) It is always in altitude preselect mode, if there is an altitude dialed into the G600 or the 3100's control head. Yes, there are some times, particularly when you have altitude hold engaged, when it is not, but other than that, if there is an altitude set it wants to capture it. The nice thing about this is it's hard to inadvertently disable altitude select, which is a common problem on Collins autopilots.
2) Most older autopilots I have used will sync to whatever pitch or vertical speed you are at when you engage the autopilot. The 3100 will not do that, it will try to pitch to whatever it is already set for. This may be a limitation of it's interface with the G600, not sure about that. So if you engage it and hit VS, it will immediately pitch to whatever is bugged on the vertical speed. This means that before you hit VS, you better know what is bugged on the vertical speed and that it is what you want.
Other than that, it's very straightforward, and flies the airplane very well. I don't have any experience with the KFC-300 in Commanders, but I would say it flies better than the Collins AP-106. And I am sure it will be much more maintainable going into the future.