A lean engine can damage itself.  That  may be what caused the crack in the turbo waste gate of my Supra.  After the AFM and injector mod, I decided I'd better monitor the exhaust gas temp.  Autometer makes a very good gauge.  The best place to put the sensor is in a location where it can monitor the exhaust gases coming from the entire engine.  There are not many good spots to do this on the exhaust manifold.  Many have compromised and installed the sensor post turbo knowing that they will have to allow for the extra heat added by the turbo. 

Since my sensor was a clamp type and not screw in, I was limited in the circumference that I could go around.  I decided to add it to the number 6 cylinder runner as I figured that this is the weakest cylinder anyway.  I started by dropping the turbo elbow (I had this off the car a few months ago for engine work and knew it wouldn't have any stuck bolts).  I used some twine to pull this back towards the firewall knowing my HKS downpipe has a flex section.  Then I remove the turbo's oil and water lines and the compressor side hoses.  If you haven't replaced the accordion hose recently, plan on buying another as it will probably be very brittle.  I carefully removed the turbo and set it aside.  Next off was the manifold itself and then its heat shields.  I simply drilled a hole in the runner and blew out the manifold with compressed air.  I also had to modify the heat shield. 

One big warning, don't hook the gauge lights up post rheostat  (dash light dimmer), it cannot take the current and will blow.

1- Removing the manifold is difficult enough that I have heard many folks talk about doing this upgrade with the manifold on the car.  I wouldn't recommend this.  I know that I would want to be absolutely sure I wasn't going to get any metal slivers into my turbo.  My job would probably have been even harder had I not had this all loose and off not too long ago.  The level of difficulty increases if there are stuck bolts and/or stripped studs.

1- Having a clamp on sensor severely limited the number of places that I could read from.  I would recommend spending the extra money for a screw in one and have a bung welded in just in front of the turbo.

2- I now want to get white gauge faces for my dash.

The payoff for this mod is hard to judge.  My car runs rich and I haven't come close to leaning out.  Of course I now have the peace of mind of knowing and not wondering.  All-in-all, I would say this or an air/fuel mixture gauge should be a requirement to anyone that has some fuel/air upgrades. 

None.  Keep your fingers crossed, I guess.

This gauge can be used to tune the engine.  Mostly it is a safety feature, and an important one.