Paris Museum Pass is a real time commitment — fit it in only if it's a priority.
Allow Pass (2/4/6 consecutive days).
If you'll hit several big sights in a few days (Louvre, Orsay, Versailles, Arc, Sainte-Chapelle), it saves real money and lets you walk past ticket queues. The catch: the Louvre and a few others still require a separate timed reservation, and it doesn't cover the Eiffel Tower.