People were clamoring for an invite because it was profoundly better than anything else at the time. I browsed the Superhuman site and I can’t personally find anything compelling.
FWIW, I do think it's an excellent email product. However, it is generally not going to be worth the money unless you send a ton of email all the time (exec, sales person, director+ eng leader).
It's pretty useless to me. I used to live in email. Now I'm this weird greybeard wondering why nobody at the company ever emails anything.