I've had two. Definitely worth a go if you like baths. Try and wait to get in, don't get in too early or else you won't feel much benefit. It's like when you've got period pains and you use a hot water bottle.
I didn't use gas and air as both my water births were home births, I wouldn't be without a pool though!
It's calmer for mum and baby and easier on your perineum. I didn't tear, and my babies were all 9lb+. With my last, I was 6cm dilated, I got in the pool and gave birth within 5 minutes. Being in the pool helped keep me upright in a good birthing position.
I honestly don't know why people don't automatically go for water birth, but everyone is different, we all like different things.
I would say, water or not, don't be on your back if you can help it! It slows things down, increases risk of complications and assisted deliveries. Always important to be in the optimum position. Your midwife can help you find a position you're most comfortable in.