Opéra de la Bastille

hmmm. I am a bit puzzled by ericp's assessment that Opera 'purists' should go to the Opera Garnier. Makes me think that he didn't see a performance or that he hasn't been to the Palais Garnier. As the son of a cellist I have been lucky enough to have been to the Opera around 30 times. The Bastille is on a par with any of the Opera houses i have ever been to, in terms of sound at least, and vastly superior to any of the Older houses I have been to such as covent Garden or the Palais Garnier, or even in Italy. What's more the design is such that the stage is visible and unobstucted from all of the seats. It is very much comparable to Salzburg. The Garnier is used primarily for Ballet in the last few years as it is not technically up to the same standards, by any stretch.
If you want a tour of a nice building or if you want to see the ballet in Paris, your best bet is probably the Garnier, similarly the focus tends to be on more traditional productions and production values when they stage Operas(though that is not always the case). If you want to see the Opera, then the better peices and the more respected artists tend to play at the Garnier. Even though I have sat up in the rafters, after queueing and would recomend this to those on a tight budget, as the sound is exactly the same as in the lower circle, I would recomend paying as much as you can afford, getting dressed up and making the most of it if you possibly can. If four hours of Verdi isn't your thing there are also smaller concerts in the salles, which are often brilliant and can be booked for 20odd euros. The Opera Garnier is fantastically romantic and I would recomend going for the Ballet. For Opera enthusiasts, there is only one choice however.

Opera Bastille is very industrial in its architecture, with exposed granite/concrete inside the theatre itself, black-colored balconies and boxes, and, acoustically, obviously not optimal for opera, as they have microphones at the lip of the stage and speakers suspended from the ceiling.
They get world-class opera singers and put on great performances, but if you're a purist, try to schedule your trip for when the opera is performing at Palais Garnier, and be sure to book your tickets online well in advance.