I babysat for a friend who had one of these once. Her baby was a bit older, getting on for a year old at the time. All was quiet. Then, after a very uneventful evening, the alarm went off.
I ran up the stairs and was confronted with an unfamiliar stair gate, alarm going off all the time. Briefly, I wondered about diving head first over the gate... I fumbled for the catch... meeeeeeeeeeep.... I pulled and pushed.... meeeeeeeep, finally the thing opened. I burst into the baby's room, to find the baby fast asleep half-on, half-off the mat. I pressed reset and went downstairs, heart beating like a drum.
This happened THREE MORE TIMES before the parents came home. I was a nervous wreck. And the last time, it woke the baby up, luckily as the parents were walking through the door.
The funniest part, though, was when she came to babysit at my house.
"Where's you baby monitor?"
"We haven't got a baby monitor?"
Cue an expression which meant she clearly thought we were mad as hatters. In our defense, we did live in a very small, very flimsy house, and the only time we failed to hear DS was when watching a documentary about Pearl Jam.
Short answer: no, I wouldn't get an Angelcare monitor, it would make me into a nervous wreck. But it's essential kit for some people.