Our son is another tummy sleeper, so we bought this to alleviate some of our concerns about his safety. I must say that having been convinced it was a product that exploited parents anxieties, I am now extremely pleased we have it and very grateful it was invented!
I had 3 concerns when I bought it..
(1) False alarms
(2) Baby crawling out of range of the pad and setting it off
(3) Picking up the baby in the middle of the night, forgetting to turn sensor off and waking husband (or whole street) with alarm!
After using it for a couple of weeks, this is the reality...
(1) There appear to be two market leaders selling sensor pads. If you check the reviews you will notice the other brand seems to have quite a few mentions of false alarms and indeed when I discussed buying one with other parents the "false alarm" complaint only came from people who had bought that brand, after using it for a while it has only once had a false alarm, thankfully during the day.
(2) When I saw the size of the pad I was quite dismayed as I was worried that as soon as the baby's chest area was out of range the alarm would sound. In reality the pad is placed in the centre of the cot and the baby has more than once crawled all the way to the end of the cot (toes just in range of pad!) and its still not gone off. As you can see from (1) & (2) it strikes the balance between being sensitive enough and too sensitive!
(3) As far as setting it off by picking the baby up, fortunately the base unit gives you a warning beep and a pause before the alarm goes off, so I have frequently scooped the baby up, heard the beep and deactivated it in time. The only time it has set the alarm off is when I picked the baby up and walked straight downstairs with him! A small price to pay.