I have two Tommee Tippee monitors with the motion sensor - one for each child! I suppose I should be but I'm not. I'm fully, fully, . Mine are the older version of this: www.tommeetippee.co.uk/product/suresound_ultimate_monitor/
I think they're the best thing I've ever bought.
My kids are perfectly healthy. When DC 1 came along my health visitor advised me against buying one. Someone he knew had one with the ticking on and it made them really paranoid - according to him.
I ignored him and bought one for DD and it has had totally the opposite effect. It's made me completely relaxed and happy. When DS came along we bought one for him too. Mine are 4.5 and 2.1 now and I don't usually turn the alarm on anymore. You can have them with ticking on or off (I think - I've never used ticking). I have always had mine set to just flashing.
I think they're great. On an evening I forget about them - occassionaly I glance up and see both kids flashing and I'm happy. Same on a night when you. I roll over, glance at each flashing monitor and I'm happy.
I used to have them on alarm when the kids were little. We had a few false alarms when they were very small. (But as PortAndLemon says, at least you know they're working) But the comfort outweighed the inconvenience, in my mind.
If the kids are poorly now I turn the alarm on - just in case. DS got his stomach fully jumped on by a 6' idiot dad (not ours!) that got onto a bouncy castle when DS was on with a couple of littlies at the weekend. I was out of my mind over the weekend. I put on the soundseek (which makes the monitor louder but you get a lot of hiss - but who cares when you're in that state?) and I managed to get some sleep as you could kind of hear it flash IYKWIM (I could have put tick on but I'd forgotton about that function).
I'm a very anxious person, compounded by a couple of scary incidents, so, to me, mine are worth their weight in gold 10x over.
The person who said you need to know what to do if you have a genuine alarm makes a really good point.
The only downside to mine is that you get a bit of interference on the parent units if you have things with transformers plugged in in the same room - not a prob in the living room but a bit hissy in the bedroom if I put my mobile on charge overnight. Having said that, I still love mine so much I don't mind.