Yes, I've had it a couple of times and would never have it during labour as it effects me terribly, makes me really drowsy and nauseous. My friend had it during labour and the nurses had a real hard time waking her to push and then she felt she couldn't enjoy her baby afterwards. But everyone is different.
I had it and suffered no side effects. I was a little woozy but not out of it, but I was feeling like that off the gas, they had to turn it down lol. I felt it helped, certainly with the contractions, but I think I had it too far into the labour because it takes a good 20mins to kick in and if I remember correctly my son was born within about 45mins. I wouldn't rule out having it again.
Pethadine is not a pain relief it I'd a sedative. I was given gas & pethadine early in my labor as my body was involuntarally pushing the baby out before I was dilated enough. It was great in the sense that it stopped this but it also made me have no urge to push with my contractions my partner & mum really had to coax me.