We put together this Quick Guide with information on OBD-II scanning, along with short reviews of a number of scanners. You can find our Quick Guide here: http://reviews.cnet.com/4520-11288_7-6441564-1.html
Check Engine error/light
Why? For two important reasons:
1) Help pin-point the area of error code problem
2) It keeps your mechanic honest
The scanner just allows you a better understanding of what's going on under the hood. Further, the sensors buried in new vehicles simply need that attention as any replacement part is costly and thus hopefully you replace the right part as appearance alone may not be telling to include some shade tree mechanics tests. Depending on what scanner you buy the simplest are under $100, better ones should cover wider vehicle models and/or offer updatability.
What experiences have you had?
off soapbox -----Willy :