Well I have worked with installing the Google Checkout mode with ZC and feel that the module does not completely integrate with ZC as it could. For example:
On a ZC site that would use the GC module without any other payment options for customers used the customer after viewing items and placing items into thier cart is still presented with both the ZC and GC checkout buttons. If the customer completes the transaction via GC and returns to the site ZC will still show the items in the cart even though payment has completed.
GC will communicate with ZC and install the new user account using GC information and will also show the order status in ZC however, GC will not modify ZC stock inventory which could lead to having to refund a customer if they purchase an out of stock item.
If a customer after completing checkout via GC would like to return to ZC to say view a downloadable file they have to log on with the information used at GC however, they would not have a password setup on ZC so, they would have to attempt to logon and click forgot password to get a password for the ZC site.
Fortunately, GC communicates with ZC so that an order that has not been paid for cannot be downloaded or seen until payment completion.
Since on a site we are using GC only I would have liked the option to remove the ZC checkout system completely since GC bypasses it anyways.
Just a few issues we have encountered with the GC module.
