Yep, me, for the last 6 or 8 months — and I drive a diesel! My mileage is typically a bit lower than the OP's, so I don't know that I've saved an awful lot, but it's a no-lose situation in that
...your Benchmark Motorcard comes with a weekly price guarantee.
This is a fixed price which ensures that wherever you use the card, this is the maximum amount that you can be charged. The only time it’s different is when the pump price is lower and you are automatically charged the cheaper of the two.
At the moment, they're giving away 5 x €4 carwash vouchers, too.
Incidentally, see their fuel-saving tips here (which echo much of the advice given above). If you live in Dublin, consider joining/forming a car pool on [broken link removed].