Rather than use a clip-on version, go with something like AngelCare's line of under-the-mattress ones. False-positives will be the bane of your existence, so you want to avoid things that can fall off. With ours we usually got alarms when our kids rolled to the very edge of the bed.
We never had any real issues with either of our kids, and only false alarms. But I know my wife got better sleep just because the alarm was there (only in part because I was usually the one to check when the alarm went off).
We stopped using it nearly immediately. It was a glorified audio only monitor for us, the check for movement/breathing was giving lots of false positives and drove us insane.
Other parents even turned on a beep for each breath, made it sound like a hospital. You subconsciously gold your breath if the device beeps with a tiny delay.. beep..beep....???beep
Same one we've used for our three. Awesome product. False alarms only really were an issue the first few weeks of life, as the babies got bigger, it was easier to pick up the breathing patterns.
We never had any real issues with either of our kids, and only false alarms. But I know my wife got better sleep just because the alarm was there (only in part because I was usually the one to check when the alarm went off).