I too Regiojet between Vienna and Brno and then Brno and Prague. Two different days but the same train number. This was last month and earlier this month, respectively.
I traveled in first class. I chose seats in the "quiet" car as I llike quiet.
Both trips were enjoyable. On one, while the "noisy" car was busy, there were only three of us in the "quiet" car. Water was handed out and if you wanted something to eat or drink you could either go to the "dining" car or order from your seat.
It was comfortable, the wifi worked well, and if I remember correctly, only one was a few minutes late.
Traveling in first I also got to use the Regiojet lounge. Not much to write home about but quieter and they had free coffee and tea.