We took this train when we visited Spain two years ago. A word of advice. Check the time on your tickets. We pre-purchased the tickets and somewhere along the way (timezone change? I'm not sure) we got the departure time wrong. We missed the train and had to pay a taxi to speed us to the next station. One hour and 100 euro later but we made it to the station with 5 minutes to spare. Thank goodness it's a slow train. It's a trip my kids will never forget.
As for the train itself, we paid for the private cabins. It comes with a dinner served in a dining car right after you leave Granada. The cabins are small. If you are a light sleeper this is not for you. This is not a high speed train but a local. It stops a dozen times along the way. The multiple stops, noise and slow speed make for a jerky ride.
If you have never taken a night train (we never had) it's a fun experience. I'm not sure I'd do it again. You arrive in the morning in Barcelona ready to go and the extra cost of the sleeper car is offset (somewhat) by the savings in one nights hotel room.
Enjoy!