I've never seen the point in them.
There's also the issue of them making it easier to get to the ignition wiring should your car get broken into, i'm not sure where insurance companys sit with these types of devices. Also is it not illegal to leave an unattended car running?
Personally i think the best turbo timer is the driver, i usually drive 'normal' before i get home after giving it a blast to allow the oil and engine to cool down.