I'll keep this simple because this shake is exactly as you'd expect. It's a typical thick frostyshake and it tastes like caramel. Now it's not crazy caramel so it's not too far crazy from middle of the road.
It's a decent looking treat and look: it's a caramel shake so there's not a ton of leeway here unless they use a real terrible caramel. It seems fine, it has a straightforward taste, though when I was done, I think I felt more dry than when I started and so I wound up washing this down with a bottle of water. Not sure how else to describe it.
I just want to say two things. One, I think it's bold for wendy's to offer a straight caramel shake because I don't think anyone else does. Two, that said, this exists because the caramel frosty parfait exists and for no other reason. I don't think there is a huge caramel shake market (though feel free to comment and prove me wrong).