Yopa etc are a bit hit and miss. I think sometimes if you need to sell quickly a traditional agent can help. When I sold my last house the first agent (traditional) got 9 viewers through the door in 16 weeks. The next got almost 60 in the same time.
A good estate agent will sell your property to buyers - actively contact people to tell them how great the property is and that it is worth looking at. Of course, many buyers only look to keep the estate agent happy!
The builders next door used Yopa, or PurpleBricks etc - they were in no rush to sell, just wanted to keep costs down and wait for somebody to offer close to asking.
The downside is if Yopa etc fail to sell, you have paid for nothing. At least with traditional agents you can change if they turn out to be useless (like my first choice).