Disclaimer: I like our Prius Plug-in, but I really don't like our Prius C. The plug-in version came with a little green sticker that lets you whiz by other people that didn't think to plop down the extra dough for a "range extended" electric (which the Prius Plug-in really is not that, but I digress).
First, a question for Prius owners: do you think the $2500-$4000 cost of a replacement batter pack at 9 years would nullify your fuel saving over that time period?
For us, the answer is mostly no, but it depends on what you drove before (or what you would drive instead). Versus the 2000
Passport (driving 350 miles a week for commuting), which has a 21 gallon tank (that we usually filled at about 16 gallons).
It would cost about $26k in fuel over 9 years (16 gallons/week * $3.50/gallon * 52 weeks/year * 9 years) versus the Prius at about $14k (8.5 gallons/week * $3.50/gallon * 52 weeks/year * 9 years).
It does put a dent in the savings though, and when compared to other high mpg cares the "win" isn't as clear. If you consider something like Ford's EcoBoost which get's good milage and performance from tiny engines, it probably would not win, this is assuming the total cost of ownership for a Ford is similar to the Toyota, but maybe that's a laughable thing to assume...
If you compare it to a Camry, the Prius doesn't seem to win. Camry is a Toyota, so you're safer in assuming the platform's reliability and TCO are similar. For me the Camry would be about $35 a week in gas, or $16k over a 9 year period, which clearly does not win over the Prius plus replacing it's battery pack.