I'm getting mine on Z-plan (same as A-plan) for what it's worth. And of course you can't justify getting an ST on economy alone.
If you can (and want to) afford it, it's a nice sporty middle of the road car that has decent fuel economy. I wouldn't get a Mustang for myself because I want to drive the car year around. In my opinion, anything else that will be this much fun would cost much more (aside from a MS3 or something). Other options in the price range would be a WRX (All wheel drive would be nice, but 25 or less mpg highway would not, and I think you're talking more initial cost than an ST anyway). What else compares for the price?