I like Alex Peper's stuff. But I must warn you all that his software is not very user friendly (in my opinion), and it doesn't look as "pretty" as products like Autoengenuity or Autotap.
Having said that, I use Peper's software because:
1. It's no-frills and it works precisely as advertised
2. He doesn't use deceptive sales tactics and misleading "facts"
3. He updates the software at least once per month, so it's always getting new features
4. It's very affordable
5. You can have direct communication with the developer (Alex)
It has nice plotting features, logging, data analysis, diagnostic tests, and so on. He offers connectors with a range of price and capabilities (I am currently using the top-of-the-line TRICAN model, having recently upgraded from the TRICOM model). You can even buy an unassembled connector and assemble it yourself as a kit (very cool). His stuff is very much for the hobbyist, whereas other products are just for your average Joe who just wants the software to tell him what to do. I'm a hobbyist; I like tweaking things myself.
What good is something like this on a diesel? I personally use it continuously on my truck to monitor %load, RPM, IAT, speed, ECT, and % throttle. I can also read and clear codes if they arise. Unfortunately, I have not been able to get more enhanced data such as injection pressure, timing, crank position signal, etc. This functionality may come around in the future. I don't know what enhanced parameters are available for 2nd gen trucks.
I've used the system on: 2002 Civic, 1997 Corolla, 2001 Sportage, and 2 2003 Cummins trucks. It has performed flawlessly on all of these.
Gassers have more available information, but that's mostly because they have a lot more complex ignition systems than diesels. On a gasser you can look at ignition timing, spark advance, and O2 sensors. Obviously these things mean nothing on a diesel.
My advice:
If you want OBD-II software that you just plug in and it spits out codes, reads some basic data, and looks pretty buy Autoengenuity or Autotap.
If you're a hobbyist who wants more functionality and customizability, get Peper's software.
I hope that helps a bit...
-Ryan