It depends on how hard you ride the bike, really.. I noticed a big difference in how solid the front end of my bike felt before I got 1 block from my house.
I ride corners pretty hard (even slow ones) and felt (and enjoyed) the difference right away..
Even more, I noticed it the first time I was riding off road, and the front end didn't deflect off of every bump, unless it was a big bump..
Is a brace needed? No, you can live without one, but I know I'd never take mine off, due to better handling on the front end of my bike.
The brace and Progressive springs, along with 7.5 weight oil have the front end of my KLR almost as good as the back end, which has a MOAB shock, built for my weight and riding style.