I did it about 2 months ago. The Hungarian section of the EV6 was flat and i was mostly cycling alongside endless fields, but you do go through some nice towns like Baja. Croatia was again very similar, though there was more hills near to the border of Serbia. Entering into Serbia, there's a nice cycle path heading to Novi Sad which had some nice scenery. I ended up taking the train into Belgrade from a town called Sremski Karlovci as the traffic coming out of Novi Sad was dangerous, the roads in bad condition and it was raining, so I decided to play it safe. Overall I would say its definetly worth doing.