I am going to have to agree with the original poster. I tinkered with the sc1200 and the ds12a's for a little while. I was never happy, and I could never stop tinkering. Finally I gave up and installed the Ademco Vista 20p. After I set it up, I never had to tinker with it again. It just works as it should.
Problems I had with X10 security:
1) DS12a reception was ridiculously bad. SC1200 is located in the middle of my first floor. I live in a rowhome and it has a completely open floor plan. No walls or doors on the entire floor except for the bathroom. All the sensors (door or window) had direct line of sight to the receiver and were placed at 10-20 feet from it. I constantly had sensor check in errors, or when I opened and closed the doors, the receiver would not receive the signal. It was extremely unreliable.
2) We always used the keyfobs, but they worked 50% of the time from right outside the door (20 feet away from the SC1200). I would have to phone in a lot of times to see if the fob's signal was received and the system was armed.
3) Again, because of the keyfob's poor reception, we had a lot of false alarms coming home. I would wave my arm around outside the door, repeatedly pressing the buttons, and sometimes it still didn't work and the alarm would go off when I opened the door.
4) Also on many days, I would wake up in the morning, the dogs would be going crazy because the SC1200 would constantly beep. Because my wife would lock the door and attempt to arm the system with the keyfob from the outside, but the SC1200 would start beeping nonstop (and also not arm) because a sensor had failed to check in during the night. She wouldn't hear it, and would assume the system was armed.
5) The annoying feedback with the speaker would keep coming back. It would require a battery pull and a power cycle, and it prevented me from mounting the unit.
6) The keyfob buttons would get pushed in my pocket all the time.
7) I tried to get external powerhorns and sirens to work with no luck. Couldn't figure out any outdoor siren options.
With the Vista 20p,
1) I have the wireless receiver in the basement, and I never had any reception problems in three months, it even works all the way from the 3rd floor, into the basement. (SC1200 didn't even work in the same room)
2) The keyfobs have been 100% reliable, the buttons are recessed and require a press and hold action that prevents accidental presses in my pocket.
3) The actual system is in the basement, even if the keypad is yanked off the wall, siren still sounds and the dialer dials out.
4) With the optional X10 transformer, you can control X10 devices.
5) On ebay, you can buy brand new door/window sensors for as low as $15
6) Compatible with a ton of sirens including cheap outdoor ones.
7) Has a ton of settings you can modify, such as creating several user codes, including guest ones, or duress codes that act like the alarm is off but it still dials out
8 ) Most importantly, since I set it up 3 months ago, I never had to tinker, reliability has been 100%, never had to phone in to make sure it was armed (although I did in the beginning frequently just to test), and never had any false alarms (unless it was due to human error such as forgetting the alarm was on and opening the window)
Sure I spent a little more (not a whole lot) but considering my time is worth something as well, I think I am well ahead in the long run. I finally have peace of mind.
I got rid of all security equipment I got from X10. The cameras got returned (Don't even get me started on those things. I replaced them with a Foscam ip cam and never looked back). SC1200, DS12a's and the keyfobs got thrown out (the only things I kept were the batteries), I just kept the lighting equipment as I found that even though the X10 wireless devices are all plagued with horrible reception, the powerline stuff actually works fine in my house.
