We booked our trip to Wien a few months in advance, and communication with Hélène and Rainer was exemplary throughout. They offered help with finding the easiest way to the apartment from the airport.
Getting to the apartment is really straightforward. There's a train line that runs from the airport to a nearby station. Also, one can take the CAT train and then transfer to a U-bahn line, which will take you to within a few hundred meters of the apartment. The place is also close to the scenic Schönbrunn, which we (and our daughter) really enjoyed.
Knowing that we were bringing our 1½-year-old, Rainer had found some picture books for her. Our hosts had also made sure to stock the fridge with a few essentials, which we really appreciated.
We wholeheartedly recommend staying at Rainer & Hélène's. When we return to Vienna, we definitely will.