Customer Rating: - Good, not great
I replaced a hard-wired alarm that was 20 years old with an SCO5 that I bought locally (and paid more). It looks identical to the SCO5CN and may be identical. The only difference I can see in the description is that the CN version apparently has two different alarms -- mine has only one (three beeps for fire, four for CO). Two would be better. (The SCO7CN talks and tells you whether you have a CO problem or a smoke problem. You need to know the difference, because in the case of CO, you want to get the heck out of there, and in the case of fire, you want to find an extinguisher and put it out.)

I disconnected the old unit and took it down, capping all the wires with screw-on plastic wire nuts. This unit nicely fit the old junction box, so screwing it in was trivial. First Alert tech support recommended covering the junction box but that wasn't practical and I can't see how it makes a difference since there's a plastic base in the new unit that covers everything.

The good news:

The SCO5 looks nice and does double duty as a CO alarm. The smoke alarm is photoelectric, which is safer (no radioactivity) and less sensitive to false alarms from steam (it's outside a bathroom and, so far, no a false alarms). It attaches easily and bayonets off easily. You can change the batteries without taking the unit down, but frankly, it's not all that easy to change the batteries while looking up at the thing and I expect that I'll take the whole thing down to do it. It uses readily available AA batteries, but apparently can't use rechargeables. You're supposed to change them once a year, and it will beep at you if the batteries are low.

The bad news:

First, the instructions are just terrible, a jumbled mess. Way too much to wade through. Small type. Disorganized. Not simple at all.

Second, the alarm is SO LOUD! First Alert recommends that you test this thing once a week! To do that, you must hold down the plastic button for 3-5 seconds. The result is 3 blasts of the alarm for smoke, followed by four blasts for CO. You better wear strong ear protection when you do this because from the distance you will be standing, it will nearly blow your eardrums out. I didn't realize how many blasts were coming (bad manual!) and took my hands away from my ears too quickly. Big mistake! If it ever goes off, you have to hold that same button down, again for three to five seconds, to shut it down. That's a long time to stand near that much noise. Holding the button down takes some pressure and in the middle of a fire, I doubt that anybody will hold it down that long. It should shut it off instantly.

Finally, there is no indication that the machine is working -- no steady light or flashing light that says "all is well, I have power." Maybe none of the battery operated units do this, I don't know. One of the LEDs is labeled "power/smoke" -- but, as far as I can tell, it does nothing regarding power.

Last, a disappointment -- I would have preferred a unit that hooks up to AC power. I've got the hookup and would have used it. But as far as I can tell, nobody makes a combo smoke/CO alarm that uses photoelectric detection and also plugs in.

I'm reasonably satisfied with this thing but I expect that if it ever goes off I'm going to be heading to an ear doctor. And the manual needs a complete rewrite for clarity and simplicity.

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Customer Rating: - Good value for replacement
Needed to replace my old fire detector and carbon monoxide unit. Got rid of both and now one small unit that was easy to install. Battery easy to replace. Price was right on Amazon and got it in one day. Great service.

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Customer Rating: - All homes should have at least two
Bought this to have an additional CO detector upstairs. So far, so good. If it works you can't complain. I guess if it doesn't work you can't complain either.

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Customer Rating: - So far so good
So far it seems to work great. Very loud alarm. Nice aesthetic design. Hopefully won't ever need it.