We had an alarm system installed when we first moved in and we've been pretty happy with it. Having located a copy of the installer's manual (and noting the installer code through close observation at the time of install) we are able to make any changes we want without involving the security company.
The peril of alarm systems is that false alarms annoy the neighbours and can result in a bill (yech). In five years we've had only a couple of false alarms; a loosely installed contact in the basement window, an oversensitive motion sensor and premature failure of one of our smoke detectors. Only one of these resulted in a bill. Our monitoring service includes free replacement of defective hardware, so we were able to get the smoke detector replaced without too much hassle. The bonus? We discovered that we have pretty awesome firetruck response in our area - the trucks arrived before we were even able to finish letting the alarm company know that there did not appear to be smoke or a fire.
Since we have a lot of people come and go in our house, having the alarm system is pretty handy. We can figure out who was home at what time, as the system keeps track and emails a report. We're even contemplated having the alarm update our phone system to redirect phone calls from home to cellphone when we leave and from cellphone to home when we come back.
When planning the system we tried to look forward towards our goals long term. Instead of limiting ourselves we spent a little extra cash and installed keypads all over the place. One by each door and one upstairs as well. We made sure that system had lots of room for expansion and have been adding to it steadily over the years. We also made sure that we could tie it back into the rest of our systems by having it network enabled.