My wife and I did this when we bought our Petworth rowhouse. We use ASG (based in Beltsville MD) and have been pretty pleased. We have two glass breaks, motion detector, and three door contacts. Our installation was $275, negotatied down from somewhere around $400+ (it IS negotiable!) and our monitoring service is $32.95. We signed a three year deal.
One thing to keep in mind, they have tried to raise my price by a coule dollars twice, I looked at my contract and it says they can raise my price by inflation and no more, and if they raise it by more, I can get out of my contract. E-mails telling them if they continued with the price increase, I expect my contract to be voided and go month-to-month quickly had them roll back the price increase, and I had a rep call me and tell me she would note that my rate cannot increase for the duration of the contract. Just something to be aware of.
Other than that, they are a local company, installation was good. We had an issue with the back door, as it was a bit too far off, they had to come out a few times to fix it, but added a repeater, and no more issues. We have the $5 service plan (included in the $32.95) and it makes it so all the visits are free. Sorry for being so long-winded. In summation, they have done a good job, and I would definitely recommend 🙂