I have the Blue Driver, and I note that what it does depends on what vehicle it is attached to. It supports a long list of "things" generically, but some of those "things" appear to be implemented differently on different vehicles, thus support for them varies. This should make sense, of course, because an ability to read a sensor your car does not have in the first place makes no sense. For instance, my X5 35d has two turbochargers, each of which have a pressure sensor, but most turbo vehicles have only one turbo, thus one sensor.
The main thing I like about the Blue Driver is that it comes with all the error codes in its data base already, there are no in-app purchases to make. So I have used it on the X5, as well as my wife's 2001 Jetta and our 2005 Jeep Libby, and it seems to work on all of them, albeit with somewhat different things available for each.
I note that there are other apps/dongles that have more or less features. For the X5, there is one called Carly for BMW that also allows the owner to do some coding, such as, for example, registering a new battery with the ECU, a handy feature which I have suggested the Blue Driver folks add in the future.
I have no experience with Scangauge products, but I do like the Blue Driver's use of my iPod Touch as the display. It allows for a pictorial "gauge" as well as real-time graphing of multiple parameters, which you can't get from a Scangauge, as well as emailing yourself an Excel spreadsheet of recorded data for further analysis.