It is difficult to draw a user experience consensus of Birger/VF because the gestation period was so protracted. It is a great pity it didn't come out of traps when the Red One was released.
It is only in the past six months that it has really come out of beta and anyone whose experience predates that would have had a less than satisfying experience. The mount itself is bombproof while the controller, being made of plastic gives the wrong impression in a professional environment, however it is in fact, very robust.
I found that when I had mastered the system it was virtually error proof - repeatable and accurate to echo's Paul's assessment. Its weakness, if it is a weakness, is that you have to take the time to learn how to use it if you want to fully exploit its functionally.
If the focus puller hasn't mastered the unit before the the shoot - s/he will suffer but a good focus puller will know the gear.
The downside is that it doesn't offer a fast lens wider than 24mm and the learning curve is steeper than I'd like. That aside, with a set of L lenses it makes a great system.
I would be very interested to hear more of Paul's experience of its limitations and advantages on those shorts.