  It has been proposed that topological insulators can be best characterized
not as surface conductors, but as bulk magnetoelectrics that -- under the right
conditions-- have a universal quantized magnetoelectric response coefficient
$e^2/2h$. However, it is not clear to what extent these conditions are
achievable in real materials that can have disorder, finite chemical potential,
residual dissipation, and even inversion symmetry. This has led to some
confusion and misconceptions. The primary goal of this work is to illustrate
exactly under what real life scenarios and in what context topological
insulators can be described as magnetoelectrics. We explore analogies of the 3D
magnetoelectric response to electric polarization in 1D in detail, the formal
vs. effective polarization and magnetoelectric susceptibility, the 1/2
quantized surface quantum Hall effect, the multivalued nature of the
magnetoelectric susceptibility, the role of inversion symmetry, the effects of
dissipation, and the necessity for finite frequency measurements. We present
these issues from the perspective of experimentalists who have struggled to
take the beautiful theoretical ideas and to try to measure their (sometimes
subtle) physical consequences in messy real material systems.
