Well, I was there 20 years ago and then it was stunning. Both Mulu Park itself, all the caves around there (Deer, Clearwater) and the Pinnacles were all extremely worthwhile. We found the bizarre insect life and the incredible trees (like this one http://www.flickr.com/photos/simonmonk/753275262/) amazing.
Back then it wasn't a particularly easy place to get to nor stay in, but I believe it's easier today. about 7 years ago
Mulu is definitely worth visiting. The caves are easy to get to, traveling on the rivers is wonderful, and the nature is unique. Be sure to see the bats emerge at dusk. The hike up to the Pinnacles is not very long but it is difficult and steep and hot but worth it. Getting there is more than half the fun. about 7 years ago