It took a while to get the system dialed in, but it works perfectly now. I think I have about four track weekends of practice on it now. This spring has been very wet; only one track weekend this year.
You can lock-up the rear with the foot lever alone; but as designed, you cannot lock-up the rear with the thumb brake without activating the front brake. (Which turns out to be a good thing.)
The thumb brake is best used for its intended purpose, changing the geometry when entering a chicane, or if you are running wide when exiting a corner, some mild trail-braking, or to prevent a wheelie when cresting a hill (or to control a wheelie).
That said, last time out I found myself starting to use it in heavy braking situations (e.g. at the end of straight before dropping into a corner) and it is amazing how well using both front and rear brakes contributes to reducing the braking distance required (thus, enabling braking later). I know that sounds obvious; I could also do that with the foot lever before I installed the system, but I never had the feel to effectively apply the rear brake without putting too much braking into the rear.
My goal this summer (if the rain ever stops) is to become more proficient in its use in a variety of applications.