Michael Y.
Yelp
First off, YOU NEED A RESERVATION. This is a VERY popular local spot, and similar to Da Enzo, you can't be upset if there's just no seat for you upon walking up.
Osteria Zi'Umberto remains my number one spot for a meal when I visit Rome. Is it the best food? It comes in a VERY close tie to Da Enzo. However, the crackling personalities and fiery spirit I've come to love about Rome can be felt here.
First off, I've seen some complaints about Mariano, the owner. He is gruff, he is direct, and if you do some research on Italian profanity, you will find that his vocabulary is very colorful. He is also my favorite part about this place. He and his staff deliver an experience that results in you eating excellent food. They know that's what they're there for. Sure, I've never had a long, pleasant conversation with the server where they answered all 15 of my questions while catering to MY native tongue. But I've never had an empty glass, I was never short on food, and I never had to worry about a bad meal. And I've had short, quippy conversations where the server applauded my 3 words of Italian. The spirit of Rome is alive here, and the authentic Roman cuisine keeps it breathing.
I don't want to tell you how much I love this place, because I want it to myself.
Never mind, don't go, it's terrible.