Greg - I tend to be a bit of a price shopper - so I would need to see price parity or something that made me feel like I was getting a good deal.  That is my left brain need. 

My right brain would likely be hooked by some disturbing question about whether I rememembered to keep my water supply at its purest.  Rather than a subscription, I might use a technique that is used in the aerospace industry of pay on performance (aka: power by the hour).  So, rather than calling it a subscription, I would call it a Britta Purity Promise Package or Britta HealthGuard Service or Britta Pure and Simple Plan...  Emphasize that people are buying the hope of crystal pure water, and that the only way that they can guarantee an ongoing supply of pure water is by establishing a water purity plan where Britta takes the risk out and the consumer can breathe (and drink) easier.  - mark