I just bought these and placed them under the sink. The alarm is very loud and the sensor is quite sensitive.

What if you aren't home to hear the alarm?

That's right jelly, it only helps if you're around to hear the alarm. But then I guess you could say the same thing about smoke alarms. Anyhow, we're usually around in the mornings and evenings, so we'd very likely hear it before too much damage was done. I guess it wouldn't work for a vacation home or if you travelled a lot though.

We have those in the laundry room and in the basement by the hot water heater and by the water softener. It has saved us many times! Washing machine tub has overflowed 3 times in the last 2 years. Those alarms work!!

I think we are going to get one for user dw and fridge.

We put one in for our new fridge and will get another for the sink when the plumbing is done.
We had a leak in the old kitchen - flooded the kitchen floor and seeped into the carpet in the living room and the family room ( on each side of the kitchen). Never again!

It's better than nothing, because we are home a lot of the time. But they are no good if you are not home. There are ones that will call your mobile phone, which I am hoping to get.

Reminder to turn off the water to the house when you travel, and the washer off after use. These can be really catastrophic.

I have a couple. One in each bathroom under the sink, one by the water heater, one by the washer, one near the top if the sump hole, and one for under the kitchen sink.

I had one with a sensor for under the DW, but I couldn't get it positioned in a helpful place. So I got a plastic pan that sets under it instead to let the water come out the front where it can be seen.

Sure they have limitations, but they are not expensive. The one in the sump hole has already saved us once, so I consider them to have paid for themselves already. And I get to tell DH I was right, not crazy :-)

I use them. After 4 small leaks in 5 years, one un-detected and discovered when we put in the new dishwasher, it is worth it to have a 'heads up'. We do check ALL plumbing regularly now, especially the past problem areas, as everyone should. Newly installed plumbing needs monitoring as well to check for a defective part or human error.
-Our new heavy duty washer hose was defective, but that was obvious during installation.

Though they're a lot more expensive, there's always the FloodStop, which actually turns off the water.

Here is a link that might be useful: FloodStop

I have one of those on my water heater, but you'd have to put quite a few in. My plumber said he won't put them in all over because they go off, the whole system goes haywire, and you can't tell what the problem is. Maybe he just doesn't like installing them. There's also only a one year warranty and I'm not sure how you know if it doesn't work anymore.