I bought one of these recently. It comes in a clear plastic blister pack with instructions, screws, mounting bracket and batteries. Out of the pack it looks just like a smoke detector, and is fitted the same way. It took a couple of minutes to install. It goes on the wall, held by two screws. You mount the bracket on the wall, and then slot the detector into it. The device can be reremoved easily if necessary, though the battery compartment is on the front panel. I fitted mine at the top of the staircase.
Just like a smoke detector, it has test and reset buttons. The green light flashes every 30 seconds so you know it is working. It is not intrusive at all.
Just as I would not live in a house without a smoke detector, I would not be without a CO detector.