The view from the top is the best part, especially if you go for sunset. The way to Mount Popa is a desert, is really hot and susty during the day. On the mountain there are locals cleaning the stairs all day long asking for tips, the temples are dirty on the other hand and there many monkeys running around. Worth to go for sure but don't expect too much except for the view.
Make sure to spend the whole day here. It's most notorious for its major 1980 eruption, the deadliest and most economically destructive volcanic event in the history of the United States.