I'm not sure which one you are referring to, but I have one and it will give you all the basic troubleshooting codes (O2 sensor, EGR, misfires, etc, etc) and some diagnostics and allows you to reset the code. It's useful. It does not provide info for Volvo proprietary codes. There's a place in Europe that is putting one out I think, other than that only the dealer has that equipment.
I would love to have one. I have had o2 sensor issues and misfires. After replacing the O2 and the coils, the car runs great! my light is still on. I took it to autozone to have the codes cleared but it came back on. As soon as someone hears about a pc based code reader, please let me know. The one that Autozone uses is $129.00 but its pretty basic.