I followed several guys who make it and there is a general consensus: it is pretty inaccurate.
Thank you, that helped.
And btw, so far I've only scouted a 5-ball player and I got a 5-ball star rating, so 100% accuracy for me. :)
But no really, even factoring the baseline inaccuracy I still think Group Demonstration is hardly worth it, unless you know you will have a very low pick and you want to spot some decent, mid-level players.
In my draft list I have 19 1 or 2-ball players, I think I'd rather spend those 10 points on 10 players with a base of 4 or 5 balls.
Still curious to hear other peoples' opinion, of course.