I attended a meeting of the local EV club, where they were showing off a couple of "plug in hybrid" models of the Prius. One of the presenters was the manager of a large municpal motor pool which featured 50+ Prii (?? Priuses?) One had been converted to "plug in". What the "plug in" part is/was intended to do is increase the battery-only range of the Prius.
The consensus among those present was that the battery-only range of a standard Prius is in the neighborhood of 5-10 miles, provided:
1. You don't "lead foot" it.
2. You don't exceed 34 mph, at which point the IC engine automatically kicks in.
3. You don't use the heater, which gets its heat from the IC engine.
The "plug in" conversion consisted of a supplemental battery pack (20 motorcycle starting batteries, or some such) to help top off the existing Prius NiMH pack. The supplemental pack gets nothing from the regen braking or normal charging system of the NiMH pack - it's just used to boost the charge of the NiMH pack.
The conversion costs between $7K and $15K, and extends the "electric only" range to around 13 miles, more or less depending on conditions and driving habits. The reason the conversion is not more integrated into the drive system of the car is that nobody wants to void their warranty. If all it does is help the original NiMH battery, Toyota is apparently fine with it.
Another interesting thing is that, in the EPA mpg test cycle (or whatever it's called), the plug in version of the Prius has substantially higher emissions. In the normal Prius, during the EPA test cycle, the IC engine might cut in 30 or so times. With the plug-in conversion, the ICE comes on maybe 6 times. The ICE's duty cycle in the plug-in converted Prius makes it quite the polluter.
Why is this, you ask? Well, when the catalytic converter is cold, it doesn't work. It has to be (very) hot. So, when the ICE in the Prius comes on, they actually inject a bit of gasolene into the catalyst to get it hot. At that time, the exhaust is very dirty. Eventually, everything gets hot enough, the injected gas is completely burned, and the catalyst does its job. With the plug in conversion, the ICE doesn't run enough to get (and keep) the catalyst at the optimum temperature.
It's my humble opinion that the Prius is a typical compromise. Yes, it gets good fuel economy, but it's not a very good electric vehicle, and it's not a good ICE vehicle. Toyota made tradeoffs in order for the car to appeal to a wider audience. (I suppose they figured people weren't ready or able to plug their cars in whenever they parked.) Also, electric vehicles have a very hard time competing with ICE for the combination of range and performance. We're getting there, but we're not there yet. We are, however, far enough along that a lot of people could get around in EVs because they don't always need the range and speed of an ICE vehicle.
A Prius gets 50-60 mpg with the usual disclaimers. A Rabbit diesel could do this in 1980. I would submit that a Prius, given an appropriate diesel engine using current technology, would get better mileage than the hybrid.
I think the Prius' appeal is a combination of fuel economy and low emissions. The design was carefully thought out for this, and tampering with the original system brings things out of balance from the intended design. As EVs move into the forefront, hybrids will fade to the back. They probably won't go away, but they'll fade a bit.
...edited for spelling, 4Apr08