It is indeed a really good one, although the Orangutans on the other side took most of my time and attention. But Melbourne Zoo deserves a lot of credit for what they've done in the past decade or so.
Well, I saw Orangutans on the other side and Siamangs on this one, but if you compare the exhibits, vice versa would seem more likely. However I know this facility is very recent, so I think that for enrichment reasons they decided to design it so that both species could be in each exhibit interchangeably; even together at times, they do it in San Diego and probably in other zoos too.
I also thought that the orangs had this space, with the siamangs on the other side, but I believe that I've read that the apes actually rotate between the two exhibits. I've never heard of Melbourne mixing the animals together, but I'd love to know if that is indeed the case.
That explains why I had seen this described as the Orangutan's outdoor enclosure when it first opened. Rotating them probably gives more interest to their lives- are there any obvious problems?
I've personally seen all three groups in all three of the exhibits but recently the siamangs have been spending a lot of the time in this exhibit and it has been looking good for it. The trees are growing and the ground cover is getting quite high.