For me, I'd like some isolation just to keep my ears from ringing afterwards.

When I can't even hear the drivetrain because of the wind noise (17-20 mph into a 15-mph headwind is fairly loud), then it's too loud, especially for two hours of riding.

I've ridden before with either isolating earbuds or good earplugs, and I'd usually feel pretty relaxed when I got home. After my ride earlier this evening without earbuds or plugs, I was sitting there on the john, noticing that my ears were still ringing -- which wasn't a problem after rides with hearing protection.