I agree, leaving the theology aside, there is a lot we could learn from the Mormons. Certainly my Catholic Church is doing an underwhelming job of just hanging on to young people, let alone engaging them.
Part of the growth of Mormonism is outreach - and a large part is demographic. They have 2.5 kids or more, while most Americans have 1.5.
Since Mormon theology leads one to Hell, their effectiveness in youth ministry is irrelevant.