I wrote the first draft of my novel without much of a plan, except for a chapter breakdown (one or two sentences to describe each chapter) and what I thought was a fair idea of who the characters were. I wrote over 50,000 words.
I set it aside for a few weeks, and read through that first draft.
The story was sluggish, the main character was weak but there were also some good ideas in there and the second main character was strong, pushing me to work on my main character to match this one.
Then I used "the Marshall plan to novel writing" book. I had read it while writing my first draft, on the recommendation of a friend, and I found it helpful to plan my second draft.
It helped me to ask myself important questions about my characters (especially my main character) and the story (especially the importance of surprises) and I then fleshed out a scene by scene plan of the second draft, loosely based on the first draft as well as everything I had learnt about my story and characters since then.
I wrote the second draft following those scenes, which describe what the goal of the protagonist of the scene is, and the goal achieved (or not achieved), and basic details such as when and where it takes place.
I found it worked well for my second draft; I am in the editing stage of my second draft, and I can say both my main characters are now strongly defined and the story holds up together.
So after trying both, I would say that planning scenes help me write; however, I have adapted my plan as I went along and some issues/solutions came up. A plan isn't a plan unless it's flexible, and all that.