I wouldn't buy them at the gas station, but most big stores, grocery stores in particular, have a huge aisle-end stand of gift cards for most everything, including all the gas, etc.
I use Mobil ones all the time (those of us in the SE know the deal at Publix - $50 gas GC for $40 when buying groceries, no fees).
Never have a problem but I always just pay at the pump.