Ya, I have an .st domain with is <my-first-name>@<first-three-letters-of-last-name>.st and I've had a few people have a lot of trouble with it. With one, it took about 6 or 7 tries to explain it to him before I just took over and typed it for him. Most people do get it, though, and sometimes think it's neat.

i have a 3-letter .onl domain that is the first 3 letters of my last name

I have actually needed to use it, because my cell provider (Visible) has absolutely moronic form validation that doesn't recognize a hyphen as a valid character in an email address, even though it very much is. (My main .net domain has a hyphen in it)

Very true, I found a similar issue with vanity/hack domains too. Picked up https://jo.hn a couple of years ago and the confusion it causes isn't really worth the cool-factor most of the time!

They tend to really just be suitable as a supplementary domain for short links and the like (youtu.be, goo.gl, etc.)