Bought one for our last house. Think it was called Response. (Bit ironic, that). Wireless system, so easy to install. External alarm with batt, internal PIR sensors and codepad, all with tamper proofing. System optionally extendable. Simple to fit. Found it reliable, not prone to false alarms once properly set up.
Only thing was, no one in our quiet rural area paid any attention to the thing going off! not the company's fault, of course, but think what you want it to do.
Better maybe to go with a monitored system, or at least one that will dial your mobile, activate a viewcam etc. Depends what you want.