The trade off is worth it.
I've done this math more than once and have each time concluded it was not worth it for me.
It comes down to the number of miles you'll drive over the period in which you want to break even on the investment, your estimate of fuel savings (which won't be as much as the OD percentage), your estimate of future fuel costs, and the time value of the money spent now versus buying fuel over time.
If you're the type who doesn't keep a vehicle very long the resale value might be another component.
If you are going to drive enough miles over the period where you need break-even it can be a good idea. I don't drive enough for it to make sense.
