I recently bought a house. I found arranging a viewing via purple bricks very difficult, and in some cases, impossible. When one of their agents (ie not the home owner) were doing the viewings, they had an 'open day' for the viewings, which was actually just an hour when the agent would be there. The website implied that you could ask for viewings at another time, but when I requested another time, I was told no. Even when I clearly said that I was not available at that time, the agents refused to offer another time. At some 'open days', because the agent had insisted in packing everyone in to a half an hour slot, it was hard to actually move around the house and see it properly, and impossible to ask the agent questions.
We did make an offer which was accepted on a purple bricks property. But the very next day, they then suddenly said they'd accepted a higher offer. We would have been willing to increase our offer, but they didn't even bother to ask us. Therefore the seller missed out on getting more for their property.
Estate agents have a bad reputation, but what they do takes skill. After we had an offer accepted through a high St agent, he was instrumental in helping keep the chain together.
You get what you pay for.