I have had 3 births all with Gas and Air and TENS.
First turned into an emergency shoulder dystocia due to mis-management by a seriously out of depth midwife. After waiting in the day unit for 40 mins and being told by duty mw that "there is no way you are in labour as you are easily able to talk and walk" - was examined and found to be 5cm. No issues with G and A after that once got to room so although birth was a nightmare (6/7 hours later, to be fair) did at least have some good pain relief.
By far the most painful one was the second one, the only one where she was giving it the old crap about "you might be hours", "it will be ages", "save the gas and air for when you really need it" at 4cm shortly after arrival. At that point she disapeared for a "tea-break" leaving me alone with DP and absolutely no pain relief and the next we saw of her was when he pushed the emergency button 45 mins later as the head was emerging
and I realised I had inadvertently given birth in excruciating agony, screaming for the epidural I was adamant I would never have, literally begging DP to knock me out and stop the pain (baby back to back) with absolutely no bloody pain relief whatsoever despite being cm's away from a bloody G and A machine that was not switched on!! 
Third time i had a lovely, caring midwife who actually let me drive the progress and took me seriously when I said it was kicking off and gave me the gas and air as soon as I arrived despite only being 2/3 cm at the time. She left it up to me as to when I started to use it, which I did when I needed to, starting slowly and building up gradually so that by the time I got to transition and crowning, I had such a good rhythm going with the gas and air and breathing and was timing it so well to peak with contraction peaks that it was actually pain free at the end - I couldn't believe it. I could feel the most violent, bone-shaking contractions as per other labours but no pain.
All of which leads me to believe that the best midwives are the ones that have the time and patience to spend with women in labour and actually listen to what they are saying about what their own bodies are telling them. They have the experience (hopefully) I know, but I think it is dangerous to make assumptions about how dilated, how long it's going to take etc. I appreciate they must encounter some women screaming at 1cm that probably need immediate epidurals but most women can be trusted to use pain relief as and when they feel the need I think.