in my experience, there are whiteboards... and there are ceramic whiteboards. anything non-ceramic is going to gunk up over time and will never become completely clean.
the ceramic ones are awesome because they erase effortlessly and resist the inevitable ghosting and staining that will happen with the cheaper ones. they are super durable and will last a lifetime.
the only downside is they are reallllly heavy and require a more thoughtful installation.