I've been putting the payment gateway portion of the Paid Subscriber add on through various tests. Running it over and over in a test environment is helpful, but there's nothing like the real thing.
Yesterday I finished putting together a proposal to skin a site. I'm in the US and the customer is in the UK. It made sense to use PayPal to accept the deposit for the project. So I put together an ASPX page for the proposal. On the page, I placed a Buy Now button for each contract option (e.g., 50% deposit, pay in full with professional discount) and tried them out in the PayPal sandbox. Then I switched the gateway to live mode and used my wife's PayPal account to put through a $1 charge. It worked!
That may seem like a paltry thing to get excited about. It is. But then you're talking about the same guy who, with help from a knowledgeable friend, rewired his home from scratch and thought it was magic when the lights actually turned on.
Anyway, it was time for the real deal. To make things easy, I put a link to the page in the acceptance portion of the proposal and sent it off to the customer. There's always the possibility that a demo bug shows up (i.e., it works just fine until you show it to somebody). Fortunately, the proposal was acceptable, the checkout process worked, and I had the opportunity to see a real payment come through.