It seems to me that this board provides mostly negative information about the Prius Plug-in, so I wanted to inject my real-world experience, which has been fantastic.
My Plug-in Prius just hit 10,000 miles. I commute about 85 freeway miles per day along the San Francisco East Bay corridor. With the commute I face, EV-only is out of the question. I have three children, so the Volt is not an option. I charge the car once per day (at home). I cannot charge at work. On weekends, I often charge twice per day and drive fewer miles.
Most of this data came from the Prius display. Because it closely matches the mileage results I've logged with Fuelly.com, I believe the data are accurate. Your MPG will vary dramatically depending upon how far you drive each day and how often you recharge.
This car has been defect-free and very pleasant to own and drive. It gets a lot of positive attention, and I love passing Porsches during commute hour.
As of 8 October 2012:
Date of purchase: 15 April 2012
Miles Driven: 10,175 miles
Gallons of Gas Consumed: 155 gallons
kWh of Electricity Consumed: 413 kWh
Overall Miles per Gallon: 65.65
Electric Miles Driven: 1,595
MPGe (electric only): 131.4 (1,595 miles / (413 kWh/34.02 gas equivalence))
Gas Miles Driven: 8,579
MPG (gas only): 55.35 (8,579/155 gallons)
133 = Recharges (based on capacity replenished)
assumption: one full recharge, including charging losses = 3.1 kWh
Total Maintenance cost: $20
Replace cabin air filter (myself): $20
Oil Changes: 1, no charge
Tire Rotations: 2, no charge
Repairs/Defects: Absolutely none
The cabin filter is supposed to be replaced every 30,000 miles, but I like clean air.