Chris Juden, technical editor of the CTC, ran drag tests on bottle, BB and hub dynamos.

"All of the generators were easier to turn than riding up a 1 in 300 slope. Another way of putting that is a rise of 18 feet per mile; and there's quite a cluster of sidewall and hub-driven models around the 1/500 line, or 10 ft per mile. If that's a hill I'm a Dutchman!" - CJ

I wouldn't use a bottle generator for P-B-P, but for commuting IMO they're a competent bit of kit.