La Nouba is ~90 minutes long, plus a ~10 minutes "pre-show" clown act (warning on this matter: don't walk in alone at the last minute or you might find yourself part of the pre-show), and no intermission.
As to "Is it worth it?" That's a personal decision. To a majority of those that go see it, it is. To some it's just ain't their style. The same goes for pop concerts, Broadway shows, etc. My standard advice is to go out and buy one of the Cirque DVDs (preferably "Quidam", "Dralion", or "Saltimbanco") and watch it with your group. That will be the best barometer as to your question. I did this (having seen some Cirque on my own on A&E) and my wife's reaction was "We're going!!!" We've been twice since 2000 and we think we're going again in June.