Hey everyone, I've been to Serbia and I wanted to give my feedback on top of the excellent op post.
The gay scene is pretty much non existing, save for a few bars with mixed clientele and a sort of open-minded attitude that is pretty much all you can get in a very conservative country like that. Forget displaying your sexuality in public and be discreet.
The guys, as mentioned, are very tall and OH MY are they hung. There's quite a lot of active escorts in Belgrade, less in other cities but still available if you really need one. Not comparable with, say, Bulgaria where every single city has a dozen escorts, but it's better than nothing.
Grindr is a very valid ally in finding escorts, unexpectedly: I had several college-aged guys contact me on the app, offering sex for around 50 euros. They were very above average good-looking and even more hung. 20+cm THICK dicks for 50 euros, yes please.
Overall Serbia is a very affordable, safe and clean. Belgrade isn't the busiest or prettiest city in Europe (quite the opposite), but the guys are worth the trip by itself if you're into tall, rough looking balcanic slavs.