My friend has the 10hz QStarz and uses it with the Race Chrono Software. He's loaded this onto his mobile phone and it's a pretty handy little unit. He has a nice mount for his phone and it can both control the unit and view the data from it without the need for a laptop at the track.
I've only tested it with drags and as Eric says it's not too far off. I personally didn't like the software overly and it kinda annoyed me that I couldn't use it with my iPhone or on my Mac but I guess that'll be an issue with most of them.
They are pretty cheap on ebay though and great value for what you pay. Pretty sure this is the model he has:
http://www.ebay.com.au/itm/Qstarz-BT-Q818XT-Bluetooth-GPS-Receiver-10Hz-Aus-Seller-/270822099764?pt=AU_Electronics_GPS&hash=item3f0e414b34
I was looking at getting myself one of the Performance Boxes, can anyone share some experiences on those?