I LOVED IT! We purchased ours almost seven years ago when our daughter was born, so they most likely have changed in some way since then. We had a nephew that was a victim of SIDS and this product gave me piece of mind and allowed me to sleep as peacefully as one can with a baby.
I don't know how it works with the sensor pad under the mattress. It doesn't seem possible that it can detect their little breath, but IT DOES! I nursed both of my children (they are now 6 1/2 and 4) so there were many times in the middle of the night that I would take them out of the crib to nurse, just get sat down and the alarm would sound because I forgot to turn it off.
You can also turn off the sensor detector and use it just for sound if you are traveling and don't want to take the whole system with you (again, I had purchased ours seven years ago, so might have changed). My brother now has it, so it has been working a long time.
There were maybe around three times that one of my children wiggled their way into a corner where the sensor pad didn't detect them and the alarm sounded. But I wouldn't let that deter you. All the positives largely outweigh this one negative. I would highly recommend purchasing the system. If you have more children, you will definitely get your money's worth. If you only use it for you daughter, I'm sure you could sell it at a garage sale in a year or two and get a good portion of your investment back.