Almost from the moment hybrid cars were re-introduced to the world by Toyota and Honda a decade ago, there have been two big questions everyone asks.
1) Will you save enough on gas to make it worth the extra money to buy it?
2) What happens if the battery dies? How much does it cost?
And so you see a lot of posts talking about these two questions again and again. The problem is no one really knows the answer until it happens. Will you save money in the long run depends on the highly variable cost of gas. When you can't tell me how much gas is going to cost tomorrow, how can I tell you whether or not you're going to make out in the next three to five years?
And as for the hybrid car battery packs, we just haven't heard about massive failures (which is a good thing) happening. And even those that do happen, the warranty usually covers it. When Honda covers the battery pack for 80,000 miles and Toyota covers it for 100,000 miles, the cost of replacement is usually covered by the car company. * Note the warranty in California is for 150,000 miles.
But, now that hybrid cars have been around for a decade, we were bound to see some issues arise that occur after the warranty:
Source: HybridCars.com - Battery Problems on First-Gen Hybrids
The real-world experiences of owners with first-generation hybrids—now clocking mileage into six-figures—will be the real test of the longevity of hybrid batteries. And the commitment from carmakers and dealerships to respond to any bona fide problems.
And, unfortunately for those owners of the first generation hybrids, that's what a few of them are starting to see. When the replacement cost can range from $4 - $6,000, it's not a light decision to make as to whether it's worth it to replace the battery.
I would like to point out that every car has it's possible issues. As one commenter points out, if your transmission breaks, it's going to cost you thousands to fix it. There are several other parts on a car that are just too expensive to even contemplate replacing if something goes seriously wrong.
And then there are those who are in the majority and our car can last for a decade or for several hundred thousands of miles without a major issue. We should count ourselves as being fortunate.
But as for the question about if you should be worried about the battery, I would say you should be, but only as much as you should worry about anything breaking on the car.