We gave them a serious look and rode several belonging to friends, but decided they weren't for us. They're ideal for the contemplated uses on paved roads around the campgrounds. A big part of our biking is taking advantage of the many improved bike trails around the country, however.
We found that even with gears, they are problematic off the pavement- even on improved gravel trails. The issue is the small wheels, which are severely affected by ruts, rocks and soft spots. We're not crazy, rough-terrain mountain bikers, but experienced riders nonetheless. And we came close to wrecks any time those small tires encountered even small obstacles.
Folders with 26" wheels would be eminently suitable for us, but we settled on full-size "hybrid" bikes for the more upright riding position, compared to other full-size bikes. And frankly, the 21 gears have proven really nice for geezerly bodies and hills. 