We originally wanted to go with ADT, but after they dropped the ball with their estimate, we decided to go with a DIY service. FrontPoint is very simple and to the point. They basically mail you a box full of wireless sensors which you place all over your house. You then call up and activate with them, and voila, you're being monitored.
One of the best parts of the FrontPoint service is that your system is completely customizable. Need 40 windows covered? No problem. Want to put a flood sensor if you're in a low lying area? They have a sensor for that too. You can even add extra main panels if you need.
I also like the online and mobile monitoring. You can basically arm or disarm your system from anywhere, and even see when doors were open and closed. Pretty cool stuff.
In the end, these guys are very reliable, cheaper than ADT, and when we needed help, they were right there giving awesome customer service on the phone. Top notch!