I did it for a number of years. It's difficult for most people because there's more that needs doing than you have time to do it in, and priorities need to be juggled without every dropping anything.
The last two exec assistant jobs I left each hired two people to do what I'd been doing alone. They were reasonably happy to have saved the money by letting me take on more and more work as long as I could function. I wasn't unhappy at either job and didn't leave either because of the work overload, but it was very different from other jobs where you work 8 to 5 with an hour lunch and when you're not working, there's nothing else work related you should be doing--no possibility of working through lunch, coming in early, staying late, or working weekends to catch up.