OK...late to this thread and didn't read every post :huh:
Elk & HAI are both comparable and deciding between the 2 will either be preference, cost or integration requirements. Personally, I chose Elk because I was able to piece it together as I go without the upfront higher cost of HAI. The beauty of these systems is how much they can do reliably without a PC needed - lighting is definitely one of them (even though I use a seperate controller). I started small and added more switches/receptacles as time went on and my needs required.
One such use of lighting is when we come home I hit a button on my keyfob - the alarm is disarmed, the garage door opens and if the time is 1 hour before sunset and 1 hour after sunrise - the lights between the garage and living room are turned on to 50% to light a pathway though the house. An important thing I do now as it's cold outside is control my son's electric blanket (his is the only one without a builtin timer) so it will automatically turn off at a set time or when the house is armed in away mode.
I did see a mention of "deafen them" - keep in mind depending on your home layout that would only be a good idea if the bedroom areas are well shieded from the sirens - especially if you use "screamers!". Otherwise keep the interior sirens to a minimum and let 'em blare when armed in away mode as you wouldn't want to blast out your own family! I have screamers all though my home, but they will only go off in away mode. A few months ago it was late at night and the house was armed in stay instant mode when my wife wasn't paying attention and opened the garage door to go to her car when the alarm instantly went off and turned on every light in the house! Needless to say, my kids woke instantly and weren't too happy - and that was only with a few of the Elk speakers.