For those of us who like to service our own cars, being able to retrieve the OBD-II codes is important. There are various readers on the market, with some more capable than others. Of course, once the codes are retrieved, you need to know how to interpret them. The common misconception is that the on-board diagnostics will tell you exactly what's wrong. The truth is, codes may be set that call out a particular component or system which are symptoms, not causes. To really get to the root of the problem, you need to interpret the information coming from your vehicle's computer. That requires years of experience and knowledge of of how everything interacts.
Or, you could just go see SAM. SAM stands for Smart Auto Management, and it's like a mechanic and an ATM rolled into one. The kiosk systems are going in at assorted chain-service shops and independent facilities. You drive your ailing car in, pay the $15 fee, plug into SAM and get a printout that is written in English even your ten-year-old will understand. Some of us will soldier on with our code readers and mental Rosetta Stones, but for folks looking for a quick answer to the "is this going to be expensive" question, SAM may be your man.