Hello, recently I saw that booking rejiojet train are way cheaper than others (almost 2x). Would rejiojet trains be a good option?
It depends on where you want to go.
Budapest to Vienna
If the timings work for you, why not? I'm not sure what the schedule looks like but the drawback of Regiojet is that they only run two trains per day, compared to the hourly ÖBB/MAV-trains. But if you don't mind that, it is hard to beat €9 for the trip.
Also, never buy train ticket from 3rd party resellers. Always buy them from the company that actually runs the trains.