I spent a night there in 2015 between Kotor and Sarajevo.
There's not much to the town, really. I wouldn't call the town center particularly charming. It's definitely worth a quick visit to the old town, maybe a little walk by the river. The highlight (one reason I visited the town at all) is the magnificent old Arslanagić Bridge, a stone bridge a short walk from the center of town. Actually, this bridge was moved, if you can believe it, piece by piece from another location a few decades ago when a valley was flooded for a reservoir. The bridge sat in pieces for a while and was eventually re-assembled in Trebinje.
The other highlight is the Serbian Orthodox monastery up on a hill overlooking the town (with a great view down). The monastery is fairly new, though - not an old structure or anything but worth checking out.
Before recommending a visit to Trebinje, I might ask where else you were planning to visit or not planning to visit? Trebinje is not far from Dubrovnik and not very touristy, so for a quick visit it might give a taste of something different without a huge time commitment. But I recommend visiting places like Bay of Kotor (especially if you have a car) instead, just because it's so beautiful. Of course, many people visit Mostar as a day trip from Dubrovnik. But if you really like old towns, Trebinje might be worth a look.