From the Hydro Tail's manufacturer: Beaker Concepts
 
The HydroTail™ is designed from the ground up to surpass current rear hydration systems in weight, aerodynamics and ease of use. By using the San Diego Low Speed Wind Tunnel it was determined that the Hydro Tail unit achieved an increase in overall speed of 1:30 to 2 min per hour when compared to the leading rear hydration company's products.
  

That’s the official word so we at Cyclespeed decided to do some of our own testing... 

We bolted two biddons plus tools to 'The Crank's' full suspension mountain bike, and sent him over to New Zealand to compete in the Motatapu 50km mountain bike challenge. 
 
After 4 hours of racing, 9 collisions with the earth (3 full impact over the handlebars) and one near fatal drowning during a creek crossing gone wrong, The Crank (minus some skin) experienced not one single bottle ejection, no damage to the Hydro Tail and no loss of equipment strapped to it. 
 
Hydro Tail - 1, New Zealand Alps - nil.  The Crank - not thirsty!  We rate the Hydro Tail highly - Buy one now.

Read The Crank's full review of this product here.
 
Prices are in Australian Dollars.