I prefer to be wrong fast, especially with new features.
We're going to get it wrong. The wrong UI, wrong flow, wrong value prop.
We won't know until a user gets it in their hands.
So get it to them fast. As fast as you can. Get from whiteboard scribbling to ugly version within a week, day, or hour if possible.
When I can't ship fast, I ask users: "Got this idea, want to try it?" If no one replies, I question whether it's the right thing.