"Back in the day" OBD-I was the way to go, because it was easy to hack.
But, OBD-II is FAR more powerful for engine management, and is now fully cracked. Plus you have real-time diagnostics with OBD-II, so you can send data out from the engine to the pits if you like, or just install a glass cluster, configured as you like. I work with a guy in Texas, who has already tweaked my DME to my desires with my stock engine. Idle speed was bumped up to reduce the neutral rattle of the tranny (light flywheel and non-sprung clutch disc), and the cam phasing comes on earlier, and smoother, for much more linear power delivery. The original BMW tune deliberately has it come "on the cam" abruptly, around 4k, so the butt-dyno can feel it. But, that is (in my opinion) sub-optimal, particularly in a corner.
So, I have a hybrid tune of Dinan stage 1, with those minor tweaks. (he may have added some other tweaks, for flavor) I'm happy with it and will go back to him to dial this in when/if I properly build this engine, which will probably be ANOTHER engine, so I can swap it in quickly. He does it over the phone, using my laptop plugged to my car. I give him remote access to the laptop and let him play while we chat, and I do as I'm told...