This past summer we hosted some HelpXers - similar system, but wider ranging in what people want to do. We really needed help with some big tasks, as both my OH and I have major health issues, so we chose our helpers very carefully from their profiles and from chatting with them via several emails (HelpX is an online system).
All four people we had here were really pleasant and interesting but we will never do it again. I seemed to spend all my time in the kitchen preparing food for them, or in the garden growing extra food for them, and my OH spent far too much time clearing up after them (tools dropped on the ground, paintbrushes left to go hard, gates left open). All in all, we felt that we would have achieved more on our own. They all did valuable work but needed to be supervised and guided - my OH has to spend a large chunk of the day sleeping and during that time disasters happened.
I think that if you like living semi-communally, or have separate accommodation and can make strict rules, and can work with the volunteers the whole time they are working, then this could be for you. But if you need time to yourself, privacy and a rest sometimes, don't do it !!
Having said that, WOOFers I understand tend to be more experienced and are more interested in the work for its own sake, whereas HelpXers just want somewhere to stay while they move around the country.
We found that our workers ideas of what they could do and what they actually could didn't match up very well. We had also set 5 hours work 5 days a week as being reasonable, but although all our workers were happy with that, they simply wouldn't go out and explore the countryside or towns after their hours had finished or on their days off - they wanted to stay with us, but do nothing - that's very annoying when you still have several hours more work to do !!
I think our problems were more to do with us than with the workers - we just don't have the extra energy any more to cope with teaching, mistakes, or the extra work involved in having guests for months on end.
Finally -there are a couple of problem helpers out there - one had to be removed from someones place by the police for being extremely abusive, and another was found to be a convicted paedophile from France, on the prowl (fortunately these were not some of our helpers I'm delighted to say).