I've spent $0 on email clients the last 20 years I've been using email. At $9/mo, if I used this service, I would have spent $2,160 on an email client.
If I imagine a scenario where someone tells me they can turn GMail into Google Inbox, I would consider a one-time $20 a fair price and could maybe be stretched to $60 with some convincing.
If I imagine a scenario where someone tells me they can turn GMail into Google Inbox, I would consider a one-time $20 a fair price and could maybe be stretched to $60 with some convincing.