You can take a look at live sites with the Joomla/X-Cart connector at [url removed, login to view] (x-cart is shown in Joomla wrapper) [url removed, login to view] (x-cart and joomla uses the same custom design we created for the customer).
Please note that the module's purpose it to provide sessions management and users databases synchronisation, so two systems will have the same set of customers and log into one system automatically leads to log into another, so an user does not have to log into two systems.
Also as a result of the integration it is possible to write small custom Joomla modules and show x-cart data on Joomla pages (for example, show featured products on main Joomla page) and vice versa.
The module does not restrict you in how you will integrate x-cart/joomla from design points of view. You can show x-cart in an iframe (using joomla wrapper component), or just create common header/footed design for x-cart/joomla and redirect customers to x-cart when they need to buy something. The module guarantees sessions continuity and accounts synchronization