My thoughts are that it's completely self-aggrandising.
They've created artificial scarcity and hype to get people to join, and they're playing off the fact they they made Basecamp, which is inexplicably popular. Basecamp has any number of usability issues. Most people I've met hate having to use it. It's not a recommendation except in that it's a recognised brand.
I say it was hyped, but I hadn't heard of it until someone posted here about making a bot to scrape invitation codes from Twitter.
Hey doesn't do anything unconventional. It has a couple of big friendly buttons instead of a menu item to make your own filter, which is how pretty much every other email client or SAAS product does it. It's also more expensive than its competitors (it's over twice the price of Protonmail for example).
One of its selling points is that you can get an @hey.com email address. To me, that's desperation to fill out a bullet list. You could say the same of any hosted email provider and really, if you're prepared to spend $100 per year, you're likely to have your own domain name for your email address anyway, one that you've been using for the last decade.
You can't "revolutionise" email by moving things around in the UI.
I think it's possible that in a year everyone's using them. Of course I think it's possible - we've seen that happen time and again with products that seemed to offer nothing - but I think it's more likely they'll be the new Yahoo! mail.
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