I was weary of spending more on this detector than the others available, but when you work it out, over a 7yr period, it's negligible. The peace of mind knowing they are there is priceless. This is a high quality product. They are guaranteed for 7years too. Then you have to replace them as the sensor only lasts that long and the battery dies. The battery is a sealed Li-on, lasts the 7yrs, so no need to replace it. I bought two for up and downstairs. I tested them both and they do alarm @ +50ppm. I didn't get any false alarms. One of them did get a 99-error, but it went when I took it out of test-mode. They have been working fine the last few months.
They are also guaranteed for the 7 years which is a credit to FireAngel. If they don't last that long, you can send it back for a replacement.
One thing I will say, the alarm isn't as loud as I thought it would be. Maybe it's just me. I don't know whether it would wake me from a deep sleep. So I've kept one of them just outside my bedroom. The other is in the kitchen. You can wall mount them, or leave them to happily sit on any flat surface slightly tilted, which is a nice touch. Comes with wall fixtures too. The green LED blips every minute to let you know it's working away too.
It is aesthetically pleasing, and the option of temperature display is good to have, but you can't have both reading on at once, it's one or the other, which is a shame. The digital readout is very easy to read. It gives real-time reading in ppm and a 28day peak history too. I like the fact that when you press the button, you're testing the alarm at the same time. Recommend it!