To everyone, much awaited the feature "email as username" added and released integrated version.
In a quick summary, there are two versions of One Page checkout, Base ($119) & Integrated (159$). You can compare the list of features at
http://www.mercuryminds.com/product/x-cart-one-page-checkout-module.html
Contact us at
sales@mercuryminds.com for any queries.
-Reva