The primary purpose of an aftermarket turbo timer is to allow your engine to idle unattended for a user specified amount of time. This will allow your engine, specifically the turbo, to return from elevated temperature levels to normal temperature levels prior to the engine being turned off. This can prevent oil coking in turbochargers that are cooling by engine oil.

Are these necessary with Subaru Turbos? No.

June 2001 TechTIPS published by Subaru for Subaru Technicians states:

"2002MY WRX TURBO COOL DOWN PROCEDURE
FHI's position regarding this is that it is not necessary to perform a "cool down/idling" procedure, as was recommended with past turbo models. Our current 2.0L turbo engine has a far greater cooling capacity and, coupled with technology advances, makes this practice no longer necessary. This explains why information about cool down is not included in the 2002MY Imprezas Owner's Manual.

The heat contained in the turbo charger will begin to vaporize the coolant at the turbo charger after the engine is stopped. This hot vapor will then enter the coolant reservoir tank which is the highest point of the coolant system. At the same time the vapor exits the turbo charger, coolant supplied from the right bank cylinder head flows into the turbo. This action cools the turbo charger down. This process will continue until the vaporizing action in the turbo charger has stopped or cooled down."

Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" turbo timers on the market. Obviously, there may have been bad timers sold, but not enough to report as "bad" overall.

Are there any downsides to turbo timers? There have not been significant amounts of problems with turbo timers. It goes without saying that repeated use of turbo timers has obvious effects on gas mileage.

Are there other uses for turbo timers? Yes. A turbo timer makes a great tool to warm up your car during cold days. Jump in, start your car, set the timer, remove your key, exit the vehicle, set the alarm, and enter a toasty vehicle 10 minutes later. It can be a more appealing and cost effective option than conventional remote starting devices. It also eliminates all thought of remote starting in gear (though there are safeties for this on remote starting equipment) for manual transmission vehicles or entering a cold vehicle because you forgot to set the climate controls prior to exiting the vehicle the night before.

Where do I buy turbo timers? Every Subaru/Import performance store sells turbo timers. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install turbo timers? Allow around 1-2 hours for install time. Professional installation, depending on your area, is around $100. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

How do I install turbo timers? Refer to the turbo timer manufacturer's instructions. For turbo timers without instructions, below is a link to some of the better known turbo timer installation instructions:
scoobymods.com instructions (with photos)
nasioc.com instructions (with photos) catered towards factory alarm users