I have it on. But I also have Hands-Free Control (jailbreak tweak). With Hands-Free I can simply say "Siri" to get Siri's attention, then when it responds speak my query. Don't have to touch my iPhone at all.Disabled to save battery life ....
I use an Activator action to switch Hands-Free on/off when I like and it automatically turns on when I plug in power and off when I switch to battery.
Any other time, all I have to do is like normal, press the Home button twice.
No hit on battery life for me. Although, Hands-Free Control uses an active listener so if you leave it on when your iPhone isn't plugged in that could impact battery life.