I used gas for about an hour really successfully with my first labour. I found it fantastic, it took the painful egde off the contractions for me and it was the only pain relief I needed.

For my second labour I used it for about 1 minute, I breathed waaaay too deeply and too quickly and I totally passed out. That was probably the weirdest feeling of my life, slipping away and then it all rushing back again. DH says I was out for about 30 seconds. I wasn't allowed to have any more gas after that but DD was born about 5 minutes later anyway.

And yes, a really important part of the experience for me was biting on the mouth piece and hearing the whirr of the ball in the chamber thingy. It gave me something to focus on.