I went to Gamzigrad-Romuliana on Thursday of last week (late July). I don't know if I would recommend Serbia in high summer, as it was over 100 degrees Fahrenheit most days. But the site itself was beautiful.

I was staying in Belgrade, and hired a driver to take me to the site instead of staying locally for the day.

The ruins themselves were beautiful. Not as spectacular as other ancient Roman sites, but Magura hill (where Galerius and Romula underwent apotheosis to become gods), was beautiful and interesting.

If you're in Belgrade, it's a three hour drive on a very open highway (not a small mountain highway like you get in western Serbia).

I have a longer review and more pictures here: UNESCO World Heritage Site: Gamzigrad-Romuliana, Palace of Galerius.