I have been lucky enough to visit here a number of times with the now defunct Orion Expeditions.
It is only a small piece of land, with the main habitat being mangroves.
This is a great place for photographing Proboscis Monkeys close up. Proboscis monkeys come into feed regularly at the various feeding stations, which have large viewing platforms. It is also good for watching their behaviours, particularly the dominant males. You can also see the females, (the ones with the cute upturned noses) and their differently coloured young.
There are other visitors, too, including other primates, such as macaques and silver leaf monkeys.
I have seen hornbills and flying paradise tree snakes.
It is located just outside of Sandakan, and day tours can visit the site.
There website is: http://www.proboscis.cc/the_sanctuary.html