After more than six months, the only problem I've had with my 2004 Prius is a dead battery. I've averaged about 52 mpg when I wasn't deliberately driving economically. As a former leadfoot Corvette owner, it took me a while to learn to get 70 mpg or even 99+ mpg, but I did accomplish this using techniques I read in the manual and online. With a car full of husky adults, patience is required for economical acceleration and hill-climbing. The typical American driver will push the pedal for more power, but the electric will kick in and save on gas. I recommend cruise control and some patience to get 50-60 mpg on that long hilly highway trip.
I enjoy the ride, especially when the engine is off and I'm rolling along silently on electric. Several times I've been cruising downhill at 70+ mph and neither electric or gas are running. Honestly, the styling makes a statement, and I enjoy showing people the technology.
A four-fold increase in the battery power would be my first choice upgrade, but at $13,500, I could buy another car! I would also like to see a light two-seat Plug-In Electric Hybrid Prius version, with a 4KW+ battery instead of 1.5KW, and a very small 1L twincam 4... would likely get 120-200+mpg