I just came back from a site59.com vacation trip to London which included flights and hotel. Total cost for 2 people was $719 or $359.50 per person after taxes (after the save59 Mastercard coupon). We got non-stop flights on AA from Boston to LHR, and 2 nights the Holiday-Inn London Bloomsbury/Russell Square. We left on 10/24 and returned on 10/27.

The mileage has already posted in our account and it was indeed under "S" as the booking code. I have no status on AA, so I can't comment on upgrade opportunities.

All in all, very easy and pleasant to work with Site59.