OK, my 2 cents:
The first level of code readers only give you the numbers, and some may only read what the generic numbers. Those are what are required by law to be available to you. take the number and plug it into Google and wala: code description. Some may come with a book to look up all the codes.
Realistically, this is all you need.
The next level would give you the code number and the "narrative" ie P0301 - Cylinder #1 Misfire
The next level of scan tool will give you freeze frame (data stored at the moment the code was set) and "live Data" (watch the engine run) Unless you know how to read this, it won't do you much good.
After that the features get more fancy, the ability to run tests, activate stuff. Manufactures scanners do even more.
Remember, a code from your car is a symptom, not a diagnosis. You go to a Dr. with a fever, he asks specific questions and comes up with a cause.
Cars are the same way, and respond differently to problems. One may give you a o2 sensor code that is a failed o2 sensor, another gives you the same code that is a faulty cat converter.
I use a auto x-ray at work, as our other scanner is Subaru specific.
Getting the code and using the internet is a good way at fixing cars.
