I have a 2008 Prius and while I echo the sentiments of most here that it is very reliable and has one of the lowest costs of ownership, I have a few gripes. I live in Utah with a lot of snow and big mountains. Those are two things the Prius does poorly with. It has very aggressive traction control that cannot be defeated. It is really easy to get stuck in situations where you wouldn't with just a little wheelspin. Steep hills are a real challenge. The 2008 model has a whopping 76 horsepower from the gasoline engine. The electric motor makes up for it around town and at the beginning of hills, but 1.6KW of battery power is very quickly exhausted when going up long hills. When driving I-70 into Denver (it is a 7% grade at times), I end up a 45 MPH with the engine running at max revs.
I haven't had to do too much outside of routine maintenance in 5 years and 80K miles of my ownership (currently at 200K). I did have to replace the brake controller, which if purchased at the dealership is a 3K part. I found a good used at a salvage yard for $75 and spent about 3 hours replacing it. My coolant control valve is now also bad which is about a 1-2 hour job and a $75.00 part. Both of these failures, especially the CCV, are fairly common on this vintage.
My father-in-law drove his 2004 for 475K miles before selling it. He was still using the original traction battery.