They normally give it to you to take the edge off when you aren't yet ready for the epi. I have never heard of it in place of the epidural, but more for relaxation during labor.
When I had it for my 2nd, it felt great and really does work for that purpose only. The only thing I can compare it to....if you ever saw Look Who's Talking?....in the beginning where she gets drugs, and the baby's hand crosses his face and he sees like a trail of fingers...that's what it's like - you're loopy!
If you haven't ever seen the movie, that was totally lost on you, but hope that helped.