Both main routes (Budapest-Szeged-Arad-Deva-Sibiu-Brasov and Budapest-Debrecen-Oradea-Cluj-Targu Mures-Brasov) are in good condition but very crowded on the Romanian part.
They both take 9-10 hours.
If you want to break the trip you could could stop at about halfway (in/around Deva on the first route respectively in Cluj on the second route) but there are plenty of accommodations all the way along these routes.
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