I have one of those too. I don't use it often, but really like it when I need it. There's a certain temperature range that it's good for. Below that range and my mix of wool and synthetic layers with a jacket over them keeps me comfortable. Above that range, I enjoy the feeling of the rain. Within the range, the rain cape prevents me from getting hypothermic with just one layer, or sweating to death if I had the full mix on.

That said, it doesn't work when I'm riding my recumbent.

I'm planning a 5-day mini tour in March, and plan to carry that rain cape with me.