Anyone who gets a lot of email and needs to stay up to date. We envisage the typical usage to be for business people, who want their work email read to them during their drive to and from work.
We think anyone who takes responsibility for their own work time will like it, such as people in leadership or management roles. Executives, self employed people, marketers, journalists, politicians, account managers, team leaders, lawyers, accountants, doctors, engineers, consultants, sales people on the road all day... lots of people who use email as a primary tool.
Inbox zero enthusiasts will particularly like the chance to use downtime to prune their inbox.