First the disclaimer. I purchased these myself. There are no affiliate links nor do any of us receive anything as a result of you reading this or clicking on links.
So what the heck are these? They're Citadel painting handles! They're designed to allow you to more easily hold your miniatures while you paint them. Instead of using glue, super glue or fun-tack you attach your figures to these instead. Here is a picture of how a variety of figures / bases attach.
I picked these up last year in December on a whim and I've been using them for a few months now. I have to say that I'm a believer. My hands are on the bigger side, so holding figures during painting was always a bit of a problem. These make it much easier. Also as they're spring loaded, no having to wait for glue to dry, nor monkeying around with fun-tack. Above is a 25mm round MDF base, a 30mm lipped plastic base as well as a 25mm x 50mm pill-shaped MDF base. No issues with any of them. I understand they'll handle up to 40mm, but that seems a stretch to me. It seems to be physically possible, but I'm not sure how the springs or plastic will stand up to that much pressure.
There are a couple of minor down-sides to them. There isn't really an easy way to deal with detached riders, unless you glue them onto a round base. So these doesn't really address that problem if you like painting riders and horses separately. They seem to be well constructed, but they are light. That means it's easy to tip them over while you have figures on top. If you're clumsy, this will be something on-going for you. But you're probably already used to that problem. Other than those two issues, I've had no reason to complain. They're not the best thing since sliced bread, but they do making painting just a little bit easier.
There are lots of reviews out there if you want to learn more before you take the plunge. I paid a bit under $35 for a box of 5 delivered through our favorite delivery service, so the price in multiples of 5 isn't really that much. I give them a solid recommendation if you're looking to spend some of your disposable income on gaming supplies.