I used it with DD1.
The first time I tried it I inhaled WAY too much and it helped with the pain but felt nauseous and dizzy. So my MW turned the mix down (they could adjust the nitrous/oxygen mix at the hospital I was in). Then I tried it again and it did nothing for me.
Later when we tried it again, with it turned up again, it was good, and I used it all through pushing.
My MW talked me through the breathing technique which from memory was inhale and count to 3 as the contraction builds then take the gas away and breathe on your own.
The good thing about it, is the effects don't last long at all. So you can try it and if it makes you feel sick or out of it, then you can just stop using it, and the sensation will pass quickly.