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Pro Support for time zones
It's easy to send emails in a different time zone using the standard time zone abbreviations such as: PST, EST, PDT, EDT etc.
Pro Has iOS app
There is a full-blown iOS app for managing or creating emails.
Con Difficult to edit emails once you've scheduled it to send later
Half the time you can't edit an email once you've scheduled. It's a very buggy function. The more reliable way to do this is to cancel the scheduling, go back to the drafts/inbox etc. Edit it as a normal draft then reschedule it. It's quite the hassle. For a paid product it shouldn't be this hard.
Con License is per Gmail account
If you want to use Boomerang with multiple Gmail accounts, you will need to pay and manage subscriptions for each Gmail account.
Con Webapp doesn't let you schedule a draft you've already made
The web app can't schedule pre-existing emails you have made in drafts. You can only schedule entirely new email content making it really awkward to have to copy and paste in a mobile app.