Used it for a few months at $30/month. Cancelled… Great concept. No competitive moat as email clients can copy the features. Looking forward to the gmail “Focus Mode”.
One of the psych principles I used to lecture about often in my keynotes was on boundaries as freeing, and discussed an experiment where children clustered around the sandbox in a playground with no fence, but used the entire playground area when there was a fence.
Parity isn't simply the availability of identical features - sometimes it's also the lack of the other features, for both the users and the developers working on the product.
That's not to say this particular instance will be a successful company. Just that the ease of copying their differentiating feature may be less of a big deal than people often think.