I used to board at a barn that has used them for years. Most would come for 3-4 weeks and then move on to their next location. The farm usually had 2-3 woofers at any one time. Just be careful about language barriers. The family that owned the farm was European and could speak several languages, I speak English, French, and some Spanish so I could usually do some translating, but the BM only spoke English and she was the one who had to deal with them everyday.
They were always very nice and eager to learn, and had all requested to be on a farm with horses. Most of them had only ever taken a few lessons when they were younger though, so they weren't much help at shows, and definitely weren't allowed to handle stallions and babies.
They always seemed to be grateful for the opportunity, during their free time they would often ask if they could set jumps or just watch me ride, or they would walk around with their cameras taking pictures of everything and anything.
I've been on the other side - travelled around the world as a wwoofer.
We tend to be a pretty awesome bunch
1) Have very clear expectations. I loved the one farm I was at where we worked longer days, but we all took days off to explore the area with the farm owner's friends. Hiking, cheese-making, wine-sampling, it's all awesome! My least favorite was when they only had a handful of hours they needed me to work... but it was so vague & irregularly timed they would get annoyed when I wanted to leave the farm.
2) Try to think of some specific projects and have a few at a time. It's more fun for everyone if there's 3-4 wwoofers, and you can get some great projects done. We canned 20 jars of applesauce (from apples that we picked), dug holes & installed a fence around their new pasture, and desensitized all of the animals to the ocean (petting zoo- she wanted beach parties!). She later built a brick oven, a cattery, and lots of other projects. It tends to work better than incorporating them too much into your daily labor (because you'll get annoyed the newbies are inefficient since they leave before they have time to get incorporated into the routine).