We had similar concerns when we got the 2001 Prius, and what I recommended was to buy the shop service manual. If you are technically inclined you can eventually service the vehicle yourself, and if not then at least you can provide the service techs where you take your vehicle to with adequate information about service and repair procedures. Not that a properly trained Toyota service technician with years of experience is of no advantage, but you can certainly ease your worries about future service if you have the proper documentation.
We tend to keep our vehicles beyond the timespan most people keep new cars for, so that's the main reason I suggested to buy the service manuals. They are not cheap, but they could be looked at as a wise investment, especially if you plan on holding on to the car for it's lifetime.
But... On the other end of the argument is reliability, and a cab driver in Vancouver bought a 2001 Prius and put ~400,000km on it by 2003, so Toyota offered him a new 2004 Prius in exchange for his very well used 2001 model, obviously he accepted their offer, and they promptly shipped his old Prius back to Japan to be dismantled and examined. To their supprise they found a lot less wear on the hybrid system, including the new fancy drivetrain than they were predicting. As far as I'm concerned if a Prius I can survive being a Taxi (some of the worst driving conditions) then I think casual drivers will indeed be very satisfied with the reliable performance of a Prius.
The most worn parts in a Prius are all in the ICE because we're still dealing with exploding gasoline vapor pushing metal parts, high temperature fluctuation and all the other various harshness involved in operating your typical hydrocarbon burning engine. That being said, the Prius is one step ahead of most cars because the ICE is much less taxed and can therefore be tuned for low emissions, less RPMs and somewhat less need for high compression. That all adds up to longer lasting ICEs in general, so I think the Prius is a much safer bet than your ordinary non-hybrid cars.
The Prius II is even more improved over the Prius I with it's larger battery and larger electric motor, so expect even better longevity out of the newer models than the first generation Prii. Toyota put a LOT of R&D into these cars, probably more R&D than any other car they ever built, and it shows, both in functionality and reliable performance, not to mention the price tag.
