The IMAP feature sounds interesting, but as someone who would be interested in exploring the service, but wouldn't be happy going "all in" with no way of switching back without losing data, it's a shame it's only planned as a feature in the higher tier.

Being able to try a service "risk free" seems really important. To start replying to emails in a new service and then lose them if it doesn't work out doesn't sound like a reasonable option.