Battery life seems very long indeed now, although there were some problems with early cars.
The first Prius, the NHW10, had a significant number of battery problems, but as this car was only ever sold new in Japan few of these are documented on a worldwide basis. The second Prius, the NHW11, had some very minor problems with battery pack sealing, but generally the batteries seem to last the life of the car. By the time the third Prius, the NHW20, was launched Toyota were confident enough to give the battery (and whole hybrid system) an 8 year, 100,000 mile warranty. I've heard of NHW20 models that have topped 200,000 miles with no problems, so wouldn't hesitate to suggest that the hybrid system is probably one of the most long-lived and reliable parts of the car.
The fourth Prius, the one that's just been launched and that is confusingly referred to as the 3rd generation by some, seems electrically very similar to the NHW 20. As such I would expect it to have similar reliability and life to the NHW20 model that it's replacing.
Overall, the later model (NHW20) Prius only has a few minor recurring problems:
- The alloy wheels are appalling. I had two sets replaced under warranty in three years, due to really bad corrosion.
- The original 12V housekeeping battery can have a relatively short life if the car isn't regularly used, as it is of small capacity and there is a modest housekeeping load drawn from it when the car is parked up. There is some evidence to suggest that higher quality/capacity aftermarket batteries last longer.
- Brake pads may well outlive brake discs at the front. My Prius was showing just 40% pad wear at 60,000 miles, but 70% disc wear.
- There was a steering recall on many NHW20s, so it's worth checking this has been done in the service record.
- Steering motors (the steering is electric) can develop a fault with the position sensor. It's relatively simple to replace, plus there is, I believe, a cheap DIY fix that's been discovered by some of the ardent Prius tweakers.
- There were some reports of a connector sealing problem on one of the sensors that occasionally caused some warning light indications. There's a simple Toyota mod to add sealant to the connector which stops the problem.
- False system warnings can occur very rarely. These often go away after a few car re-boots, as they seem to be caused by the odd signal glitch.
- Paint chipping on the front and bonnet area is common, due to the shape of the car. I doubt this is any worse than any other tear-drop shaped modern car though.