I find the remote start feature useful to warm the car engine up in the cold winter morning when the outside temperature is below freezing.
Yes you will certainly void the warranty if the dealer is not the one performs the installation.
In my case I installed the ebay brand alarm/start/trunk release kit by myself back in 2004 on my 96 Accord and it has been working fine.
If you choose to install the kit yourself, you may want to pay attention on 1. it's a lot more difficult trying to install the remote start on a car with manual transmission. Most aftermarket kits are only designed for the automatic transmission; 2. if your car has a security chip/immobilizer or the car key has the embedded chip that is required for the car to start, you will need to look into the bypass module to get the remote start to work.
You likely also need accessory parts including relay(s), diodes, hood pins, brackets, car wiring manual and others to complete the installation.