It reminds me of a similar service, ZeroMail  (also from Australia). The UI for Fluent is intuitive, and a nice break from Gmail (esp. with Gmail's new look).
Given the influx of e-mails I get a day from listservs, friends, business contacts, customers and random services (Groupon et al.), I see this already busy interface getting cluttered very fast. That said, I only tried the demo and would be interested to see what it looks like on my actual inbox with real people.