We had the good fortune to attend the opera last night at the Opera House (not all operas are presented in this venue). Tosca by Puccini was the program, an opera I'd not seen performed.
The orchestra and singers were first-rate, the acoustics of this beautiful hall was excellent, and we secured loge box seats, as seniors, for about $30 US each. Unbelievable; comparable seats at Lincoln Center couldn't be had for ten times the price. I'll admit that Tosca isn't on my list of personal favorites, but magical nonetheless.
If you're visiting Prague and have even a slight fondness for opera, please go. It's a marvelous experience.
Final note: we're in Europe with carry-on luggage only; I wore jet black tech fabric cargo pants, a dark polo shirt and sneakers, while my wife had a tech golf skirt, polo and her usual Hermès scarf to fancy things up a tad. We were in fact seriously underdressed, but many others in the audience were casually attired, and we were not treated with any condescension.