I have one. I should post a longer review at some point. It works great for email, sending/ receiving. As they tell you, external sites treat your email as spam at first because it's coming from a new domain, you have to mark you email as not spam.
I wish it had a web page like interface like gmail. I suspect they didn't do that for security. Also I'd like more support for basically hosting domains on it - I have multiple domains and get email and that all works reliably. I'd like to have an easy way to setup other web page references on the domains and host simple pages I setup. You can get them to configure your dns to add more sites but I haven't bothered. I'd like an easy to use setup. The big negative is they will host your name servers for your domain, not you.
You have to use the app on your cell phone to administer it. A web interface would be much easier to work with.
Android and ios phones have built in apps, uses what looks like good security (tls etc). It has tons of space on it. I have a catchall account on the main domain, so junkmail@<mydomain>.com goes to it. I have bought a few domains over time and I put them on their too. I can't figure out how to setup the catchall account.
If you want personal email server, this is great, works reliably.