“And I’ve got a couple of diesel Benz’s sitting in the driveway right now…”
Me too, and this might be a good time to find some deals an grab a couple more. (-;
I have driven a Prius for three years now. It has over 60,000 miles on it. Most of the mileage is highway driving. I reliably average 43 to 44 miles per gallon. The Prius is a great, trouble free car, and if you learn how to drive it it will pay you back in good mileage. It is also a comfortable road trip car, as I learned over the past Christmas holiday. We drove from Texas to New Mexico, around in Albuquerque in snow, and up to Taos in snow and ice. Three adults and a load of luggage and Christmas gifts. We averaged 42 miles per gallon for the trip.
Just found this discussion board, so if anyone is still interested in this topic, here is my 2 cents…
My '05 Prius consistently gets 50 mpg on highways, on long distance trips, when I use my cruise control and drive at about 68 mph. In the city it’s more like an average (over all the seasons) of 38 to 40 mpg’s. I have always wondered why I get better mileage on the highway than in the city. I try to do all the recommended things while driving in the city, like taking my foot off the gas to cruise towards a red light, not using the air or heat when not necessary, etc. Anyway…I’m very happy with the mileage I get on long drives on the highway. 
BLE above has just about right. The Prius uses an Atkinson-cycle engine, not the standard Otto-cycle engine. You shut the intake valves partway through the compression stroke, which keeps the compression down to what regular gas will tolerate, but allows you longer expansion on the power stroke (thus letting you extract more energy from the gasoline). That, combined with the load-leveling that you get from the hybrid system, means more efficient travel at highway speeds.
You can find the most and least efficient new cars on the fueleconomy.gov website, here:
http://www.fueleconomy.gov/feg/bestworst.shtml
As of the 2009 model year, the only car that got the same highway mileage as the Prius (45) was the Honda Civic Hybrid. That’s a smaller car. Nothing else with gasoline tops 40. The VW Jetta diesel is listed there at 41 mpg highway.
Be aware that the 2010 Prius (available now) gets 50 mpg overall, 48 mpg on the highway, per the fueleconomy.gov website. The 2010 Prius also lets you select which software mode you want for driving (sport, normal, eco). Several credible reports have said the average driver will get 60 mpg overall from a Prius in “eco” mode (where the software is optimized for fuel economy). I don’t know whether that affects highway mileage or not.
Anyway, 2010 Prius, 48 mpg highway. That’s the standard of comparison.
On the battery issue, they ought to last the life of the car, for most cars. That’s what Toyota says. And that’s what the experience to date suggests. In some states the warranty on the battery is 150,000 miles. Toyota couldn’t do that unless most of them last way beyond that. When the US national labs tested the Prius, they stopped the battery test at 180,000 miles because they didn’t see any appreciable degradation of the battery. You can screw up the battery, but you really have to try to do it – same concept as driving your car when the red oil pressure light is on. And if you do, you can pick one up on Ebay pretty much any day of the week for $600 or so. (There’s a surplus of them because the number available from wrecks exceeds the number that have required replacement). Brand new, they are currently $2440 from the dealer (plus installation, of course). I worried about it when my wife got a Prius (2005), but I haven’t given it a thought since.
Just pinged ebay – there’s a whole bunch today, anywhere from $400 to $600. That’s the “buy it now” price for the current (Gen II) model. Maybe that’ll change at some point, but right now all the data says this is just not a problem.
We have two Priuses and a hybrid Camry. Our commutes are only about 15 miles each way, and in the summer in the daylight those commutes yield about 54 mpg in the Priuses. But we have made many 500 mile highway trips between Cleveland and Williamsburg, and we usually get at least 45 mpg in the Priuses, except for the time I hit 105mph to get around a long line of slow traffic in the left lane of the three lane highway. The Camry is averaging 38mpg except when I’m pulling a trailer. While you may find conventional vehicles that’ll give you 45mpg for your commute, the Prius still has advantages when you get into the stop-and-go urban areas. My only complaints with either of the Priuses were the OE Goodyear Integrity tires that only lasted 20,000 miles per set. I quickly replaced them with Michelin Hydroedges.
I drive an 04 Prius with 90K miles and have owned it since it was new.
On the highway I get 45-50mpg and got 50 at 72mph for 175 miles. The Prius is indeed a midsize car with plenty of room with the rear seats folded flat. I drove the car 900 miles on the road over a 5 day period recently and it is a dream to drive on the road. Wind noise is non existent and it is quick in passing. I looked down to find myself going 90+ when passing one time and did not have any sense of going that fast. There is a lot of mis information being batted around out there with lots of spirited discussions about people who have never even driven one. Prius owners are kind of a cult.
I will be glad to answer any questions. I am a retired project cost engineer.