Doing a homestay gives you the opportunity to experience the daily life of people who live on the banks of the Kinabatangan river, fishing and harvesting for a living. Their houses are built along the river, using the simplest materials: expect the floor to be shaking when anyone walks in the house, and expect to bump into a python if you wander out at night…
The homestay experience was great all in all: there are some downsides, such as lots of free time (get some books and be ready to spend a few hours in a hammock reading), not-so-great activities (the “cooking lesson” boiled down to chopping big pieces of pumpkin into smaller pieces), a sometimes varying command of English in the family, but that’s what makes the experience a memorable one. If you can, I would recommend going with a friend, as it makes the whole experience better if you’ve got someone to share the idle times with.
As to the wildlife, the other comments are pretty clear about it: it simply is amazing. During a three-day-two-night stay, I saw pygmy elephants three times, standing as close as 10 meters to them, all sorts of hornbills, storks, kingfishers, apes (gibbons, macaques, orangutangs), snakes… if you can, ask for a night trip on the river, it really is worth it.
I was taken care of by a super-friendly guide all the time (who spoke English very well), so this was no problem at all.