Well, that's really up to you, but if you're wondering whether GM's pricey new plug-in hybrid will pay for itself in gas savings, Daniel Indiviglio over at The Atlantic does some back-of-the-envelope scribbling. According to GM (and these numbers aren't yet verified), the Volt will get the equivalent of 230 miles per gallon in the city. But with a likely price tag of $40,000, you'd have to drive at least 200,000 miles—or 158,000 miles if you get the $7,500 government rebate—before you save as much money with the Volt as, say, buying a Toyota Corolla.
So, no, the first-generation Volt's probably aren't the most economical purchases around. Then again, The Atlantic analysis is also betting that gas will merely rise to $3 per gallon by 2011. If it turns out the analysts at McKinsey and Goldman Sachs who are predicting much, much higher prices are right, then electric cars start to look like a better deal. I'd also quibble with Indiviglio's estimate a bit and say that the "mileage" can vary depending on how you use the car: Since the Volt can travel 40 miles on a single electrical charge before the gasoline engine kicks in, it's a lot more economical if it's used mostly for short, daily commutes. (More on that from Kathleen Pender.)
In any case, the initial round of Volt-buyers presumably aren't going to look at things purely from a cost-benefit analysis. People buy all sorts of fancy cars that don't "pay for themselves"—they do it for prestige reasons, or to assuage their environmental guilt, or who knows why. The burning question is whether there are enough wealthy people out there who will purchase the Volt and help the price tumble down as production ramps up.
Or maybe they'll all flock to competitors: Nissan is now laughing off GM's claims and bragging that its all-electric car gets the equivalent of 367 miles per gallon. Now, at this point, using the mpg metric is a bit absurd since Nissan's Leaf doesn't even use gasoline. As Sharon Silke Carty argues in USA Today, eventually we'll have to retire "miles per gallon" and start using something like "price per mile."
P.S. Another good question prospective buyers will be asking—where will you charge the darn thing? According to The New York Times, GM and Nissan are still trying to figure out what to do for people who don't have garages with electrical outlets. "If you're going to park it on the street," says GM CEO Fritz Henderson, "I don't know what to do, actually." Odds are, those drivers may have to wait until electric cars—and hence the charging infrastructure—become more widespread.