Meh, those things work, but I guarantee I could bypass that thing or any other alarm just by finding the cut starter wires and re-connecting them. The bad thing about those Ravelco pieces is that it doesn't incorporate any other anti-theft methods other than a starter kill. If a thief breaks into your car, no alarm goes off, and he has plenty of time to figure out how to bypass the Ravelco as long as no one is around. If you go with, say, a Viper alarm, they will have the same starter kill relay installed as the Ravelco, but also monitor your doors, hood, trunk, ect. Once a thief gets under the dash, he has limited time to work with because a siren is drawing attention to the car he's in.
My recommendation would be to get a basic Viper alarm system, like the 350HV, then add a battery back-up and a motion sensor if you want to keep it safe when the top/doors are off. A setup like this will probably set you back anywhere from $200-500 depending on if you want the battery back-up and the motion sensor or not.