The one way pricing is a major bonus. They are (generally) lovely people, and yes the treatment should continue at the airports and on the flights (although I can't remember how much of their ground services are subbed out at MAD...). And you will get lounge access (again, last time I was through MAD - which was a couple of years ago - it was a contract lounge rather than a BD Diamond Club lounge, but was very nice all the same).