For the past decade I had just regular rain paints that would cover the whole legs. Together with a rainproof jacket and rainproof shoes (or shoe covers) this would protect me from the rain completely. However, it would not necessarily keep me completely dry. Although the rain paints that I have feature a membrane, one would still sweat in them.
In Bonn usually it does not rain very hard or very long. Also one often has a gradual fade-in, which makes it hard to decide what to wear. If I puts on the rain pants too early I will sweat a lot, too late will have my pants wet and then it does not make sense to put on the rain pants at all. Or I start from home and it rains a lot, so I start with the rain pants. But five minutes into the ride it stops raining and I am not sure whether it will start again later. So I either keep it on and sweat, or take it off and get wet later. Or take it off and put it back on. It is a hassle every time.
I've seen the Rainlegs product on the internet years ago, and every now and then saw people using it. Eventually I had so much positive feedback from users that I decided to buy my own pair. I got them in white reflective as I do not want to ride around in neon yellow.
My experiences have been mixed, let me elaborate.
When it is just raining mildly, they do a really great job. The thighs get much more rain than the lower legs. I wear shoe covers over the shoes to prevent water getting into there. The lower legs get sprayed a bit by the rain and also water dropping off the Rainlegs. As I usually wear light pants, that dries off rather quickly.
The Rainlegs look pretty ridiculous, but one can put them on very quickly. One just snaps the straps around hip and each thigh and then also the hook-and-loop fastener near the knees. With the rain pants I needed to take off my shoes to put them on easily, otherwise it would be really though to get around the heel. There is therefore less friction in putting them on. Also they only cover the top part, so excessive heat from cycling can still vent through the bottom of the thighs, making me sweat way less.
They become problematic when it rains much stronger. With significant rain on a 40 minute commute I had soaking lower pant legs and needed to change when I got home. With dry shoes and dry thighs one could still have it dry while wearing it, but it is unpleasant.
For the seemingly unpredictable weather in Bonn they are a really good compromise, though. It is not as unpleasant to wear them in light rain, and having to occasionally bear lower pant legs wet with rain is better than to have the whole pants wet with sweat.