What a wonderful little device !!!
Being the caretaker of a rental property with a temperamental boiler in a nearby unit, a way to monitor it was needed.
Problem solved in the best possible manner.
Placed the sensor in a central location after it & the base unit had 'paired', went back to my desk - and behold - temperature monitoring is now possible anytime, 24/7, with ease.
Now if the boiler goes out again, it will be easy to know there is a problem before any possibility of a freeze-up.
Because this works so well, I may get another just to place that sensor outside as well.

Received item and tested right away. Put in brand new alkaline batteries. Placed sensor outside and reciever/readout away from digital devices. It only displayed the indoor temperature after 25 mins. I changed the batteries to rechargeable lithium batteries as suggested for colder weather, although it was only about 40 degrees that day outside and 69 degrees indoors. Tested again and left the sensor in various locations within the specified distance as per the instructions. Still no outdoor readout displayed. Either this is a defective item or just poorly made. You get what you pay for. Returned product and spent a little more on another indoor/outdoor wireless temperature sensor. This one also gives you the humidity indoors and outdoors. Much happier and it was only a few bucks more. It might have just been a bad unit but I'm glad I returned and found another that works and provides more information for a few dollars more.

These require 4 AAA batteries! This is the perfect size for our apartment and has been working well. It helps us know when we can turn the AC/heat on and off to maintain a good temperature in our home. Also useful for knowing the weather outside, especially as I head out the door to work. Definitely useful and worth the buy. Great price.

Edited: The 3rd one lasted only a few months, which is not sufficient for the price. Original review:Worked for about 18 months, which seems reasonable for the conditions it is subjected to in my climate, dying after an especially violent storm. I found some rust on the contacts, so I took a stab at drying it out with a hairdryer, which yielded an ant and 2 earwigs (all alive), but no outdoor temperature transmission. I've had similar products that lasted about the same amount of time. I just ordered another one. Less than a dollar a month for an accurate indoor-outdoor thermometer that survived 1 brutal winter, 2 very hot summers, and an unknown number of creepy-crawlers seems reasonable.

I purchased this as a gift for my mom. She loves it, especially in the heat that we've been experiencing lately. She now knows when it's too hot to even open the door to see how hot it is outside! As an elderly person, she's better off staying indoors when the weather's extreme, and this helps her stay safe.