The mailboxes are very inefficient, being regular mboxes with an index. I'd rather have the e-mails in a sqlite database. A Maildir on disk is a waste of inodes and a liability when one does a search or archives the thing. Old mail is rarely touched and sqlite has a full text search function.
The interface is okay. Quirky, dated but okay. I just need a snooze option like Gmail's but one that doesn't hide the e-mail. Now I have to convert the e-mail to a task and add an alarm to the task which requires a lot of clicks and setting options that are unergonomic.