That looks like one way to do it, but there is a better way. A switch is really not necessary here, particularly a two case switch, which is almost never preferable to a simple if statement. And even that is not necessary here. Let us use the Input object.

var aimCamera : Camera;

function Update(){
    if(Input.GetButtonDown("Fire2")){
        aimCamera.depth = 5;
        //anything else you want to have happen on aim button being pressed
    }
    if(Input.GetButtonUp("Fire2")){
        aimCamera.depth = -20;
        //anything else you want to have happen when aim is released
    }
}

Much cleaner, and more straightforward. If you so desire, the GetButtonDown and Up could be replaced with some comparisons with GetAxis, but I would not recommend it.