After CARS announced they'd be allowing these devices, I ordered mine right away.  And it just arrived today!  Here's a quick review:
First impression is that it's a nice looking unit.

A test fit in the car with the belts tightened down, and you really don't notice the device at all.  It's like it becomes part of the seat.  At first I thought I felt some pressure down on my shoulders from the c/f, but then I realized I was only feeling my harnesses.  It really does just vanish, like it's not even there.

I noticed that SSG sometimes got in the way when trying to close the quick disconnects.  Which slowed me down trying to attach everything.  But that should improve with practice.

After adjusting the tethers to fit, I could turn my head and have a full field of vision.  I could turn far enough to see the B-pillars.  I'm glad now to have the SSG, when I tilted my head to the side, that was fully tight before my helmet touched the rollcage (which has been an ongoing problem in my car).  I figure those are a decent alternative to having a seat with the side head supports.

So those are my first impressions.  My first event with it will be Lanark this weekend.  So we'll see how that goes!