These are great for resistance training and if used correctly can improve your paddling indurance.
If you get one, get it from Lee at
Fit2Surf.com. He has contributed some
surfing fitness articles to this site, and
sent a cord for review. He's also the author of
Complete Guide to Surf Fitness, and has worked with top surf athletes Ben Skinner and Oli Adams, and developed fitness coaching with the U18 British surf team.