I currently looking at a 2003 Celica GT-S w/auto trans that's for sale. From everything that I've read, the GT-S model requires premium fuel, which is a big turn-off with today's gas prices. Does anyone have a GT-S that they run regular or mid-grade fuel in? If so, any performance/mpg issues?

Even if it requires 93 octane, it will still run fine on 87 or 89. When it starts to get preignition, the computer will sense this and it will retard the ignition timing. As long as you dont drive like a maniac, it wont hurt the engine.
Just know that the power wont be quite as snappy as it should be and you wont get the best possible fuel economy.
The cost difference from 87 to 93 isnt that big of deal. If the car has a 20-gallon tank, its a difference of about $4 per fillup. If thats such a big deal to you, perhaps you shouldnt buy a GT-S. Toyota made a standard Celica that had the 1.4L from the Corolla. It didnt make as much power as the VVTL-i engine that the GT-S had, but it also takes a skilled driver to keep the GT-S' engine in the narrow rpm range where it makes that extra power. Any other time, it doesnt make any more power than does the standard Celica. Something to consider...

Good info...thanks for the help.