We have a system - used to have ADT, now have Guardian, but they are basically the same. They even use the same brand of equipment.
We like it. Very early on, we had a sensor that was bad and we had a few false alarms, but ADT came out and readjusted it. We set it off ourselves every now and then, but you have about 10 seconds to turn it off after an accidental alarm before it automatically calls the police. We always get it off that quickly.
Yes, the sensors are somewhat noticable. They aren't huge, and they are white so if you have white trim, they blend in. If you don't have white trim, they would be noticable.
I do not have window sensors, and most of the back of my house is a wall of windows. But my floorplan is pretty open, and I have a motion sensor that faces those windows so if anyone tried to come in by breaking them, the motion sensor would trigger the alarm. It was a better and less expensive option for us. Our motion sensor also 'sees' the bottom of our stairs so no one could get upstairs to the bedroom without triggering it, even if they avoided the windows. This would not work if you have pets that are free in your house though, because your pets would be triggering the alarm at night and when you aren't home.
UPDATE: For the motion sensor - we have two settings. A 'home' setting and an 'away' setting. The home setting activates everything except for the motion sensor. We use that when we are in the house and awake (for example, in the evening before bed). The 'away' setting activates everything, including the motion sensor. We use the 'away' setting when we are gone of course but also when we are sleeping. So yes, our motion sensor is on at night. There are 2 ways to do this. We used to have a key fob and we would just take that to our bedroom at night. We could then turn the system on and off from our bedroom. The key fob is an extra monthly charge though, so after a while, we decided to get another control panel installed upstairs in our house so we can turn it on and off from there now.