BLS in the area I used to work was able to use Entonox, and we used to carry it on the ALS trucks as well. However, we didn't use it that often. I think perhaps it was a little underused. A few problems come up with it, as you may have seen:
* The pain relief only lasts while the patient's inhaling the nitrous oxide, so once you arrive at a receiving facility, you either have to leave the bottle with the patient (impractical), or take it away (unethical). So usually you end up giving morphine (or another opiate) anyway.
* Some people just don't like it. There can be a lot of nausea / dizziness / dysphoria.
* There's a risk that the nitrous oxide migrates into air-filled spaces, making it dangerous in bowel obstruction, and you can't really use it in most major trauma, because it can worsen pneumothoraces (or other disease processes involving trapped air).
* It just isn't as effective as opiates in most patients. So if the pain is severe, again, you're adding opiates.
Now personally, I think that in combination with an opiate, it's a good analgesic for transport / extrication providing none of the issues above apply. It allows you to provide added analgesia to someone while you're moving them around and exposing them to vibration and stress during transport, which can then be d/c'd upon arrival, once these stressors are removed.
[For what's it's worth, I also prefer morphine over fentanyl in situations where hemodynamics aren't an issue, as I've had too many experiences where I've got the patient transferred to a bed, given report to an RN, and then had the fentanyl wear off, and been put in a situation where I feel an ethical obligation to provide further pain control, but know that if I do it in the ER, after transferring care, that I'm putting myself in a potentially risky position.]