My Tobii 4C was just delivered today, so I downloaded and started trying out Project Iris. While not as accurate as I'd hoped, I have to say I'm still impressed. Especially with the FPS mode. I'll be picking it up officially as soon as my monthly cheque comes in.
KinesicMouse is the next thing I want to try. I've been using Eviacam thus far for head tracking, but it doesn't offer the features that KM does. I'm hoping I can set it up, maybe combined with Iris, to be less stressful on my neck than Eviacam's face tracking, but even if it doesn't it will still make games a lot more playable with the expression controls. But it will have to wait until I have a bit more money.