I have had the Viper 791XV - Auto starter/Alarm Installed for a year now.
Pros:
1. Starting the car from in the house or office when it is cold as hell (which it gets up here) Car stays locked during this time which is better than having to run out start the car and then watch it from inside the house. I have my car in a garage so I put a dryer vent in the side of the garage, when I get home at night and it if it is going to be cold the next morning, I put the dryer vent hose over the exhaust and it vents out when I start the truck. Avoids the old suicide look from the garage with fumes building up and I don't have to have the garage door open which invites unwanted people.
Pro/Con: middle ground
1. Alarm; I didn't really care about the alarm, wanted just the car starter but the Viper 791 had the two way remote that let's me know if the 4R actually started if I am sitting in my office. If the alarm ever actually keeps someone from stealing my car or even attempting to break a window than that is a plus.
2. Warm up time, always a hot debate on the time taken to warm up a vehicle, will it cause more damage to sit and idle etc. vs jumping in a cold vehicle and just driving. who knows, but I like to at least start it for a few minutes if it is really cold outside. This helps out a lot of your windows are frosted up, either you sit there and wait for the windows to defrost or you jump in and off you go.
3. warning that idiot in the parking lot who parked close to your nice 4R and is about to open his door into the side of your truck. Had it happen a few times when walking out of a store to my truck and from across the parking lot I can just tell someone is about to open there car door up into my truck, so I quickly start the truck and their head usually goes on a swivel and they are then aware they better not slam their door into your truck. Can also be accomplished by just unlocking the vehicle, the beeping gets them to look as well.
4. Had to use the valet key to be mounted in the truck, so down to 1 backup key.
Cons:
1. Viper remote replaces the unit that comes with the 4R. Had to program the remote to still be able to put the rear window down. Cool thing is it is programmed now to put the rear window up or down.
2. Batteries for the remote only last 2-3 weeks. I keep a supply of AAA batteries in my car.
3. Losing the Viper remote, if I lose the remote it is something like $150 to get another one.
4. Burn through a little more gas while it idles, and during the winter I used the auto starter a lot, don't use it much in the summer except when warning those idiots in the parking lot.