Within the OBDâ??II standard, there are three protocols. You have to have an OBDâ??II system which supports the same protocol that the car to which you are connecting it uses. The three protocols are:
- PWM â?? used by most Ford vehicles
- VPW â?? Used by most General Motors vehicles
- ISO â?? used by almost everthing else
  Looking at the item on eBay, it appears that it comes with support for VPW, but that if you want support for PWM or ISO, you need to by a separate item to add that support.
  I have the Alex Peper unit, which has already been referenced in this thread, and I will add my own voice to the recommendation made in its favor. I have the 3-Combo unit, which supports all three protocols; he also offers cheaper units which support only your choice of one or two out of the three.
  As for the utility of getting one of these PC-based units instead of the $49.99 code reader from Harbor Freight: The latter unit will read the trouble code, but won't tell you anything else. WIth the Alex Peper unit, at least, and most likely with nearly any other PC-based unit, you can retrieve much more data than just the trouble codes, and I have, on a few occasions, found this additional data to be very helpful in diagnosing problems which turned out to be something other than what the trouble code itself indicated. For example, I was, at some point, getting codes that indicated that my O2 sensor was going bad. On examining the data from my O2 sensor, I was able to see patterns that indicated that it wasn't really the sensor itself, but an electrical connector that was failing to make a good connection.  Instead of having to replace the O2 sensor, I only needed to clean up that one connector.
You can also read RPMS, vehicle speed, throttle position, timing, etc. 
I can use it whenever I want. 