I will say that the boom is a great learning tool, and I've never seen the kid who learned on one who wasn't dying to get behind the boat like Mom and Dad.
That said, the sideways stance of a wakeboard seems very awkward with a fixed bar. We've had much better luck teaching kids to wakeboard behind the boat. Have them put the board against the back of the platform, get them to ball up, and pull the rope to get them a feel for the way the board will feel when they should start to stand up. Hook them to the tower or extended pole (if you have one) on a short rope so you can talk to them from the back of the boat as they're getting up. Once they're up and riding, a little quick coaching, and the next time give them a little more rope.
This is a small sample size and your results may vary.
I spent most of my money on booze, broads and boats. The rest I wasted. - Elmore Leonard