My boyfriend and I spent 2 weeks here, definitely the hightlight of our 4.5 months in South America.
The animals are the reason to come here, but the humans make it easy to stay. The owners, Vicky and Marcello must have seen hundreds and hundreds of volunteers in the 10+ years since they accidentally started an animal refuge, but they still treat you like family. That's why some volunteers stay for months, or come back for a year!
Volunteers get a lot of interaction with the free roaming spider, howler and capuchin monkeys as well as lots of macaws and parrots (who have learned some interesting words and noises, such as "donde esta el fuego?" or miaowing like a cat). There are also other animals we'd never heard of but really enoyed: 2 tayras (weasel family), a coati (like a pretty, red anteater) and a jochi (a large striped rodent) that needed walking at night. There are also a couple of wild cats, 2 spectacled bears, a cayman, a boa constrictor, and lots of turtles and tortoises and others I'm forgetting.
If you are the kind of person who needs a lot of contact with home you should make the effort to get a Bolivian sim card (you need a passport and reasonable Spanish, or get lucky in the shop with an English speaking service rep).
The sand flies can be pretty fierce and you can't wear DEET near the animals, so bring some natural repellents and citronella wrist bands.
You're provided with volunteer shirts (in varying degrees of repair - they get quite a workout), but you wear your own pants... get a couple of pairs of long cotton ones in La Paz if you don't have them (we found fake Colombias and some seconds stalls cheap).
Also, if the rainy season has started you'll be happy to have a decent poncho and rubber boots - you'll still be feeding animals and cleaning cages twice a day rain, hail or shine.
It can be dirty and physical work, so maybe not for you if you're a princess (but if so, you can just come stay at the eco cabins or tree house for a night or two - you won't meet all the animals, but you'll get a good feel of the place, and probably get some spider monkey love).