In my case, I felt the factory system was insufficient. I had a Prestige system installed. This is made by Audiovox, and has a two-way transmitter/reciever remote. (It lets you know if you are within the distance range if the alarm has gone off.
A nice bonus is that this system has a option for a remote starter, which I also had installed. Works great.
Altogether around $500 or so installed.
(FYI - The FCC recently had these 'recalled' because the transmitting distance was too far! They did not pull these out of stores, so if you can find one that still says 1000', you've got one of the good ones, IMO. The new ones will be around 500')