Nitrous is a beautiful thing, if you can get it. It's just such a logistical nightmare that few systems carry it anymore.
Pros: Works faster than morphine. Stops working as soon as you take it off. Is patient titrated, so you don't have to worry about giving too much. No allergies. Low if any hemodynamic effects. Doesn't require an IV.
Cons: Limited to 50 percent concentration in the U.S. (except in very rare, very advanced systems), which may not be enough in many cases. Is heavy and very inconvenient to carry, so it rarely leaves the ambulance, meaning it is unavailable on the scene. Pt. has to be fully co-operative for it to be used. The demand valve mask is uncomfortable for many patients, who feel smothered by it and won't tolerate it. One of the biggest cons, and the reason that a lot of systems drop it from their service, is because of the abuse potential. There are simply too many immature, unprofessional people in U.S. EMS who will play with it like it's a toy, wasting it.
In the U.S., Nitrous Oxide has to be mixed at the time of use, from separate oxygen and Nitrous cylinders, which makes the apparatus extremely expensive. In Canada, Australia, and Europe, Nitrous Oxide is available in a single, pre-mixed cylinder, making it a lot easier to carry and dispense.
Overall, I love having Nitrous available. I find it especially useful in orthopedic trauma. But given a couple of narcotic options, I can certainly live without it.