I just completed my BRC class over the weekend. I hadn't ridden in years so I opted for the basic beginner class instead of the returning rider because I thought it was better to underestimate my skills than to overestimate. Between enrollment and actually taking the class I managed to put 4000 miles on my bike.
Most of the class was pretty remedial but I still managed to fill in some gaps in my knowledge that I probably wouldn't have figured out on my own.
I had a blast watching people who had never ridden get bit by the biking bug, aced the skills test, and I will be a better rider because of it.
I would strongly recommend formal training to ANYONE, regardless of their experience in the saddle.