I can speak with experience of having received multiple voucher payouts from them.
The frequency of inbound surveys into your email inbox is great; you'll get some nearly every day. For this reason valuedopinions is my most used survey site, and barely any others that I've seen can compete with it.
The reason it isn't getting 5 stars is because I've commonly been sent surveys that I'm not eligible for, which is often infuriating. An example being that I'm in Northern Ireland, but surveys that I have received haven't given an option for residents living there.