My 2 cents..
I got just about everything X10 offers in January after reading pages and pages of feedback, reviews, etc. I ended up spending over a $1,000 dls and to this date, I am still having TONS of issues. I've spent hours and hours trying to debug and understand what's going on. Some things now work, but many still don't.
I went for the home automation "everything", 8 cameras, 7 floodlights, 2 DS7000 (one for backup), 16 window sensors (turned out not to be enough), about 10 transceivers, chimes, motion sensors (eagle, eye, alarm), all the software they offer, and much more!
In retrospect, I should have just invested a bit more and bought a "real" security system and should have looked for a "real" home automation system - not something from the 1970's that can't deal with today's houses without so much adaptation.
If price is your driving factor, then it is cheap. Just remember, you get what you pay for. At the end, "cheap" is actually more expensive - from my personal experience.
With all that said, if you're the techie type of person, and you have lots of time in your hands, this is the system for you because, honestly, it does get addictive. Once you get one thing going, you want something else to work so you expand!
If you're just after the alarm, you need to consider the following:
* the powerhorns do sound loud IF YOU'RE STANDING NEXT TO THEM. Of course, depending on the size of your house, this may or may not be an issue. I have 5 large powerhorns, 1 small powerhorn, and the system's alarm. When my system went on the other day, my friends laughed and called it "loud crickets". We were in the house watching the superbowl. Granted, my house is over 3000 square feet. On the other hand, I have a friend who got the 99 dollar package and uses it for his condominium and loves it (about 600 square feet.) I'm still looking for an outdoor siren. I've bought one, but still have no way to get it going reliably, so I use ANOTHER alarm system (built into the house) as a backup.
* if you decide on using lights as part of your alarm system (to get them to flash on/off, which I highly recommend), you will need to invest more than what the "total" package includes. Very likely, you will need noise filters, replace your lamp modules with ones that support flashing, use expensive lights (that waste a lot of energy because the basic X10 system doesn't deal with compact fluorescent lights, which is what I had in my entire house.) Yes, I had to get filters for all of the circuits where I wanted to keep those lights instead of paying 300+ a month for electricity. oh, and I got high amp filters for my gaming computer, large TV, fridge, etc. They were all blocking the DS7000 from turning on/off the lights and powerhorns. Yes, even your furnace or your cable signal may block the DS7000 from turning on/off lights
* the DS7000 is very flimsy. The first day I got mine, the 10 cent antenna on it snapped. I had to replace it with a VHF rabbit antenna (30 dollar). Good thing because now, it can reach all my window sensors (again, it has to do with the size of the house.)
* you'll need to secure the alarm remote when you leave it behind. yes, you already know, there's no keycode panel. Personally, I had to buy a lock box (50 dls more), which I bolted down to a secure place
* the keychain remote is HUGE!!! - twice the size of a car remote keychain (uses 2 AAA batteries). If you put it in your pocket, you will most likely activate the system more than once. I can't personally can't stand it. I have 5 of them, but don't ever use any!
hmm.. I'm sure I can go on and on, but I think this is enough for now.
Good luck,
Frank