The CIM iframe works great but lacks some display options. For example, I use it at a newspaper where the billing and shipping info are both useful to have. Unfortunately we cannot change the name of "Shipping" to delivery. In the case of a newspaper, this might imply we will mail the subscription which is not the case. It would also be nice to be able show or hide the shipping field if it wasn't needed.
The iframe should also support a responsively designed site. It will position further down on the page by default when viewed on a mobile device.