It has worked out really well for me, I also have the widget button to enable on line bookings through my Facebook and website and they only take 60p for each booking made that way. I think 20% is fair as its a client who wouldn't have come otherwise. People have travelled from quite a distance so they are not passing trade. We have had quite a few repeat bookings too so it has been worth having.
I have my settings on 3 hours minimum notice & 3weeks max notice and get text notifications so not had any probs with double bookings, just need to remember to update regularly. I block out time in their calendar for when I am busy on a daily basis so it only shows my free slots. I also only discount on my quiet times and will prob take discounts off completely next month until the new year.
I have found that more people pre pay direct now, I Think its cos they send emails with a £10 off code as I have had a few emails but never use it for myself!
I get at least 1 booking a week from them and my existing clients also like using the booking widget which doesn't cost me a lot.
The downside is it takes a couple of weeks for money to go in your account.
For clients that have changed their mind about the service, e.g have a cut as well, I just add the extra and they paid me but didn't change it on wahanda but if it's a lesser amount you can amend the amount on wahanda before you complete the checkout, I have managed to do it a few times with no probs.
I seem to be getting loads more emails from similar companies lately. I did sign up with pamper pod ages ago but got nothing from it, and as wahanda is working for me with just the free listing, I won't bother going with any others. They spend a lot on their marketing and even have a big advert at our local train station plus the TV adverts so think they are good at getting people to use them. I can't really think of a reason I would stop using them at the moment.