Fitting in both cases was pretty easy and I would say any competent person could fit either providing in the case of the mains model you are replacing an existing mains unit. The design is clean and they look good in most rooms, it would however of made fitting much easier if they were round so you did not have to align them with the axis of the room. When fitting the mains version its a bit difficult in some cases to get the cable to come through the ceiling in the right place other than that fitting is easy. I also like the fact they include push on Wago connects and the screws.
Setup is well thought out and I like the uses of a QR code to save you entering the device information. It did however take some time for my phone to connect to it and for it to then connect to my Wi-Fi, in fact it took several attempts. Once connected however it seems to have stayed so with no problems. One thing to note is that you do require a smart phone or an Android or IOS tablet to configure them, it is not possible to do it from a pc or mac.
Day to Day
On a day to day basis the Nest Protect is very easy to live with. The quick green glow of the "Nightly promise" is surprisingly reassuring, and the Pathlight gives just enough light to stop you from bumping into things. The nice big button is easy to hit with a broom if your detector is out of reach to silence or test. I also love the idea that it will shutdown my boiler if CO is detected although the boiler will still come on for hot water.
So far thankfully I have not had the alarm sound for either Smoke or CO other than during a test, hopefully I am not tempting fate. All in all I am very happy with my purchase, and look forward to see what Nest come out with next.