I bought this for my 4.5 year old son.  
He had great fun with it when the first time on.
(Like chasing the snake in the bottom of the pool.) Since it is heavy, it will go down to the deepest part of the pool.  Make sure your kid can swim and dive for a long time before give him this toy.  It dive through 6 swimming lanes and my son is in hot pursude....
With the baby subbies, it slow down enought for my son to catch it.

Great fun and great idea, except it leaks water badly.  On the second time, my wife found out that the internal was soaked with water.  Battery/Motor are all rusted. On the third time, the motor is dead.

With current leaking situation in the big swim pool.  I will only recommand use it in the tub  (Then what's the point of putting in the engine if it can only be used in a small tub.)