I recently looked into these for mainly our car, but the truck as a second use. The issue seems to be what level of codes you can read. Most all of them will read the "generic" codes that all cars/light trucks conforming to the OBDII spec since 1996 use. However, the manufacturers then have the "enhanced" codes that may or may not be readable by tools available to the general public. Fortunately, Ford, GM, and Chrysler also provide the enhanced codes to the general market, for the most part. The only issue then becomes, like ddillman said, what can be read from the diesels. While most of the "public" products can read the manufacturers codes from the big 3, diesels are not always supported, or completely supported, anyway. Certainly, 24valve trucks will have more to offer than 12valve ones. In any case, contact the maker of whatever product you are considering and check what their product can read off our trucks.
Also, the dealership versions can actually manipulate the engine, transmission, etc. of the vehicle while "on the test stand" through the ODBII system, so I understand. But this capability is not generally available in the public versions.
On the plus side, these things can log real time data from the sensors and such and keep a log on your computer (or palm PC, as some use) as you drive. That's cool! I'm wondering what you could learn doing this while doing a dyno run.
Do your homework. Search the web. Contact the manufacturers. Tell us what you learn and do!
-Jay
Attention: TDR Forum Junkies 
