There have been a few members who had malleable (also know as "one-piece", "semi-rigid") implants. Even (at least) one member who has had both (one at a time, of course
).
They are not "erect all the time". The implant is the same size in volume all the time. Your spongiosum tissues, when engorged, will make your penis a bit fatter/girthier when you are aroused. They are stiff all the time, but can be bent down, up, to the side or curved. Think of a piece of lead wire or single-strand copper electrical wire. You bend it into any shape you want at it stays in that shape.
Your penis can be hidden in your pants or even when you are nude, but it will not dangle naturally. If you point it down and to the left but lay down on your right side, it will defy gravity and stay left. When you are dressed, you point your penis where you want to be and it will stay there.
Deploying for urination can be problematic because the penis' length is constant and men usually can pull it out of their pants' fly in a soft, bendy state. More difficult if it is not flexible. Think of how difficult it can be to pull an erect penis out of your pants. Probably have to undo your belt altogether to urinate.
One convenience is that there is no pump. Another is that there is almost no chance of the implant failing (pump failure, leakage, etc). One danger is that, since the implant is constantly the same size, it compresses the penile tissues, especially in the glans, so blood flow can be restricted which presents a danger of tissue death and erosion. Surgeons know this risk and are careful to size a malleable to minimize that risk.
One man with a malleable who is an actor in sexually explicit films likes malleable better than inflatable because the appearance of a pump in his scrotum is not helpful to his career.
I do not have a malleable, but have corresponded a few times with a couple of members who have and read past posts (sorry, I do not have links at this time). What I have written is what I recall from what they wrote.