I think that as the season progresses team's ball magnets will advance and improve to the point where if you don't have one you won't be a contender, because someone will be able to do your job better than you.
That said let me share a quote from during build season.
Me-"So how well does the ball magnet work?"
Mentor Tim-"Well, it works really well going forward."
Me-"Soooo, then whats the point of having one? Couldn't we just push it?
Mentor Tim-
silence
Me-
silence
Mentor Tim-
smirks
Anyways, the point is that I've seen a lot of ball magnets that hardly do anything. So, if your going to put the time into making one, make sure you know exactly how your going to make it and how its going to work. Don't just go at it blind. Good Luck
