I am using one now and LOVE the peice of mind it gives me. They also have a similar kind called a snuza which apparently can be used anywhere cause it clips to LOs diaper so you can use it in the swing, RnP, car seat etc
Ditto this, except beware of using the Snuza in swing or car seat if it is moving. As a movement monitor it will detect that movement, not necessarily just baby breathing. It would be nice if you let the baby sleep in those things when not moving, though! We have used the Angelcare with all 3 and it is the only way I can sleep at night, knowing it will go off if the baby stops breathing!