The downside to the lock-on feature is -- yep, you guessed it -- it makes the game easier. A LOT easier. Without it, it would have been too hard, and not nearly as fun, but with it, you'll exit the game almost as fast as you came.
It's just an hour or so of hitting the trigger button. It's tiring, while being mildly entertaining...but mostly just tiring. It's the type of game that is very, very easy to put down and walk away from, because you just don't want to hold the gun any longer.