I only take long distance trains for fun. Flying is way more convenient at all other times. Show up 45 minutes before the domestic flight and get there at least 5 times faster.
I live in Madrid and I can travel to any major city in the country by train in less than 3 hours. I don't have to show up early. I depart from the city centre. I arrive at the city centre. 500% more convenient and (mostly) cheaper.