This is exactly my point though, get the tune only if you're after more power, and if more power would make the car more fun for you.
If you're after fuel economy, well, while it won't technically hurt, it won't help either. And it will hurt if you end up pushing harder because of the insane laughter that comes bubbling up from you every time you do... those laughs come at a price.
I say anyone who has to ask, at this point, should strongly consider NOT getting the tune. Yes, it's tons more power, yes it lets your other mods actually do something other than make some funny noises, but surely the three hundred other threads on here saying that should be sufficient to let you know that much.
If you're asking if it will make your colors brighter and your whites whiter, no it won't. If you're asking if it will make beer simultaneously less filling and more tasty, no it won't. If you're asking if it will make your car go three hundred miles per gallon and leap the snake river canyon while attracting tall leggy girls in short cuttoffs, nope, not gonna do that either. It's just gonna turn your 1.4T engine into the equivalent of a 2.0T engine, while letting you continue to get the gas mileage of a 1.4T engine if you can keep your foot out of it.
If that's not worth $300 to you, then it isn't. No shame in that, in fact more power to you (so to speak. Less power to you, in point of fact, but you know what I mean) if you're happy driving around in a stock Sonic. It's a great car as is.