I bought and installed one in my 493" RB based motor. The primary reason I went for it was to gain the tuneable vacuum advance feature.
The nice thing about it is the quick and easy altering of the timing adjustments including initial, total mechanical and vacuum advance. The unit also shows where the timing is in real time while you are driving, which greatly aids dialing in vacuum advance and total timing.
I currently have mine set to crank at 10*, idle at 22* @ 850 rpm, to have full mechanical advance of 35* at 940 rpm on up and to start adding 7* of vacuum advance at 1410 rpm with it all in at 1500 rpm. I do have the vacuum advance hooked up to manifold vacuum, so that it falls away when the throttle is opened to accelerate leaving the timing at 35* again.
My motor was the happy with the MSD Pro-Billet locked out at 35* total, but having no vacuum advance made it less efficient on the road. The motor loves the additional vacuum advance timing, gets a touch better mileage and allowed me to test and tune dozens of settings in real time before ending up with my current table using a few suggestions from AndyF.
Until this unit came out, having this ignition tuning ease & flexibility had been reserved for the EFI guys running ignition controllers. It's a good option to those of us running carbs, but needing a more advanced ignition tuning ability.
