We use X-cart and Stone Edge and its integrates reasonably "ok". There are some (annoying) caveats to the process:
* GC often denies a credit card transaction after its already gotten an order number from the cart. There's no way to connect this deny back to the order management system, meaning that a human being has to go verify that all charges are good before shipping.
* Google's oddball asynchronous mechanism means that the order management system can't interact directly with Google Checkout. So, if you want to authorize and charge on ship, we have to do that manually. If you want to send shipping notifications we have to login to GC and do that manually as well.
Its all very annoying. I got the impression that Google didn't bother to look at how merchants actually need to interact with them to get running quickly, easily and efficiently and just decided to come up with a solution that was convenient for them.
We're using them, for now. Our current integration won't scale, so we'll end up dropping them in the next 12 months or so if they don't get their issues resolved.