We have used the same one for all three of our dc's, the latest being dd (now 16 months) after lending it to my sil for her ds.
I almost didn't use it with dd as I am much more laid back with dc number 3 and we never needed it with the boys.
BUT I am so grateful that we did.
At 5 weeks old dd was sleeping in her crib right up against my side of the bed. Suddenly we were woken by the alarm going off. She in a crib rather than a cot, so I knew she hadn't wriggled off the pad. I sat up and looked at her and she was blue all around her mouth with blue lips and blue hands.
I grabbed her and tried gentle resuscitation (tummy and back rubbing, blowing on her face etc) - nothing. Dh took her and loosened her clothes then repeated what I'd done - again nothing. He panicked, passed her back to me and as he did so she finally took a breath. It felt like hours, but in reality she was only not breathing for about a minute and a half.
If we hadn't had that monitor I am convinced my beautiful girl wouldn't be here now.
After that we purchased a snuza babe, which is one of the ones that clips onto the nappy waistband and this too has given us much reassurance over the past twelve months, as it not only monitors her breathing in her carseat and pushchair it also gently vibrates before sounding the alarm just in case she is sleeping really deeply and just needs to be gently stirred.
Fortunately for us, it only ever happened the one time - but then again if we hadn't had the monitor once could have been enough.
I think the pad monitor cost us about £65 back in 2002 and the snuza cost about £70 early in 2009. Imho they are both worth their weight in gold.
As for noticing them stopping making their usual little noises etc, neither dh or I woke until the alarm went off. We were both in deep sleeps and I honestly don't think we would have known a thing about it had we not had the alarm.