Rob --

Having just traveled on a Delta-booked ticket, I can tell you that I was treated like any other Upper Class passenger -- just no limo service (but Heathrow Express to Paddington Station works just fine). You can check availability and reserve your ticket before actually transfering the miles. Delta gives you two weeks to actually purchase the ticket (that is, visiting a Delta office and signing over the miles), so you will have plenty of time to transfer miles before you have to actually "purchase".