For me the opera Garnier is one of the most beautiful sight in Paris and absolutely worth a visit.
The tickets are (in my opinion) cheap. Student's and people under 26 only pay € 6,00. The whole architekture is just breathtaking. I'm sorry that we couldn't see the ballett or a oper - it would have made my day.
But even without a performance it was the personal highlight of my week in Paris and I would definitely visit it again.
The only con I found was the mass of people/tourits there. But it was to be expected.