Since I read a ton of reviews before making my purchase, I figured I'd give back to the forum a little.
I recently bought a home security package featuring the SC1200 "Brain" alarm unit, a bunch of DS12A door/window sensors, and some other stuff including keychain remotes. I gather from a lot of other posts that a lot of people purchase this unit to help protect their property when they're not home -- for me, it is the opposite, I want to be alerted if I AM home and someone decides they want to break in. From what I could see, the SC1200 featured an entry/exit chime, which is great during the day, and also had Home and Away modes for securing the house when I'm home our when I'm out. I also liked that it featured the dialer feature.
Hooking up the SC1200 was simple -- just plug it in. Per the instructions, I placed the device in a central location in my house so none of the remote sensors are exceedingly far away. Pairing with the DS12As was also simple, just put in the batteries, turn them on, set the SC1200 to "Install" mode, and trip the sensor in the DS12A. I have a decent sized house (about 2300 sq ft, 3 floors including basement, drywall walls) and I have had no range issues. All the sensors seem to work where I need them to. Installing the DS12As was also dead simple, just screw it on. I have molding on my door frames which I thought would be an issue, but it was no problem. There's a gap of 1-1.5" between the DS12A and the magnet on the door, but that doesn't seem to be an issue and, so far, it has registered properly 100% of the time.
Unfortunately I could not use the built-in sensor with all my DS12As because some of my doors did not have space to mount the sensor, or, in the case of my sliding glass door, I didn't want to screw into the metal. For these instances I mounted an external magnetic switch and connected it to a remote DS12A. It is nice that you can add a second external sensor. Following the guidance of some members of the board here, I ordered a few SM-431-T/W Seco-Larm external magnetic switches. I wasn't sure what type of wire to use to connect between the switches and the DS12A, so I used some phone cord. This combination worked, but phone cord was probably not the best choice as it was a real pain in the ass to strip the wires. Setting this up was probably the most time consuming part of the whole process, but it is working now and has been working well.
I only really have two complaints with the system so far. The first is that the built in alarm sound is not loud enough. To try and fix this, I bought a SH10A external siren. Unfortunately that isn't very loud either, but it does work. My second complaint is that you can't call in and arm/disarm the system via phone if you have an answering machine. It does work if you disable the answering machine, but despite what the instructions say, it will not work in conjunction with an answering machine. These two issues are not deal-breakers for me by any means, and for the price I paid for the system I'm very happy with what I got. I'd recommend this to any DIY-er who wants to play around with setting up an alarm. I think as long as you don't come into it with the expectation that this system is going to match a multi-thousand dollar system from ADT, you should be happy with the results.