When you need to calculate an angle from a Vector2 structure, you can use this piece of code:

public static class Vector2Helper
{
    public static float CalculateAngle(Vector2 v)
    {
        float angle = 0.0f;

        if(v != Vector2.Zero)
        {
            v.Normalize();

            angle = (float)Math.Acos(v.Y);

            if(v.X < 0.0f)
            angle = -angle;
        }

        return angle;
    }
}

I used this to calculate an angle from the Vector2 of the Left Stick.

The original credit for this source code comes from here.