There was a thread here on the Cafe a couple of years ago on SimpliSafe. I think it was pretty informative. A lot can change in two years though. There is also a new system out called "Blink" that people are saying good things about.
I've used first generation Simplisafe devices for several years now and have been happy with them. I'm not sure what else to say except that they do what they're supposed to do and the few interactions I've had with customer support were good. I don't use monitoring however.