For my money, the sweet spot for a Brompton is 1-5 mile rides as part of a commute. Upthread there's links to people who tour on them, which is cool. I've done a 7000 mile bike tour, and I'm not sure I'd trade a touring bike for a folder for that kind of use. If I only had a Brompton, I'd try it, but I own, uh, three (3) other bikes.
Besides the ride comfort from the small wheels, it really does ride a lot like a regular bike. The ride comfort is a huge compromise, to be sure, but if you can ride a bike, getting on a Brompton takes basically zero adjustment. The steering isn't at all twitchy, and while they note that standing to pedal might feel weird, in my experience, it isn't.