elizabuff, i got one too, but not till about a month or 2 later....one night whilst baby was sleeping in with me, i suddenly woke up realising i couldnt hear the rhythym of breathing, and it was awful and what felt like probably 10mins was probably only a minute but i swear if i wasnt sleeping right next to him i wouldnt have heard the breathing stop...doctor said we will never ever know if the baby would have just started breathing again or by me waking him up from his deep sleep i stopped him from something worse....
...after that we bought a sensor mat, and i was really assured then, and i felt i could sleep properly.....but baby didnt always like sleeping in his cot, so in the end he slept with me, and you cant use the mat then cos obviously it wouldnt work the same!
what im trying to say is, its good for peace of mind, especially if you want to put baby in cot whilst you and dh spend quality time downstairs, so i would recommend it.
but your not a bad mother if you dont buy it!! couldnt your sister lend hers to you for a few months?
good luck what ever you decide, but dont forget, the market out there will have all sorts of gadgets, some good and some bad, but they know that new mothers will tend to feel guilty and buy a lot of stuff cos they know how to pull your strings